ELECTRIC 
WIRING  AND  LIGHTING 


A  HANDBOOK  OF  APPROVED  MODERN  METHODS 

OF    LIGHTING    BY    ELECTRICITY,    AND    OF 

INSTALLING     CONDUCTORS    FOR    THE 

TRANSMISSION  AND  UTILIZATION 

OF  ELECTRICITY  FOR  POWER, 

LIGHTING,     HEATING, 

AND    OTHER     USES 


Part  I— ELECTRIC  WIRING 

By  CHARLES  E.  I^NOX,  E.  E. 

CONSULTING  ELECTRICAL  ENGINEER 


Part  II— ELECTRIC  LIGHTING 

By  GEORGE  E.  SHAAD,  E.  E. 

PROFESSOR  OF  ELECTRICAL  ENGINEERING,  UNIVERSITY  OF  KANSAS 


ILLUSTRATED 


AMERICAN  TECHNICAL  SOCIETY 
CHICAGO 

1916 


Copyright  1908,  1913  by 
AMERICAN  TECHNICAL  SOCIETY 

Copyright  Great  Britain 
AH  Rights  Reserved 


CONTENTS 


PART  I 
ELECTRIC  WIRING 

PAGE 

Wires  run  concealed  in  conduits 1 

Wires  run  in  rigid  conduit 1 

Wires  run  in  flexible  metal  conduit 4 

Wires  run  in  moulding 9 

Fibrous  tubing 15 

Wires  run  exposed  on  insulators 16 

Two-wire  and  three-wire  systems 20 

Calculation  of  sizes  of  conductors 25 

Planning  a  wiring  installation 29 

Wiring  methods 29 

Wiring  systems 30 

Location  of  outlets 30 

Location  of  distributing  centers 31 

Branch  circuits ."*". 32 

Voltage  drop  in  conductors 34 

Feeders  and  mains 36 

Testing  circuits 36 

Alternating-current  circuits 40 

Comparison  with  direct-current  circuits 40 

Skin  effect 42 

Mutual  induction 42 

Line  capacity 43 

Calculation  of  drop  in  a.  c.  lines 44 

Single-phase  circuits  48 

Polyphase  circuits   53 

346638 


CONTENTS 

PAGE 

Wiring  an  office  building 54. 

Current  supply '. .  .  54 

Switchboard    55 

Load    ' 55 

Feeders  and  mains 55 

Wiring  plans  for  eacli  floor :..... 56 

Interconnection  system   58 

Outlet-boxes,  cut-out  panels  and  accessories 63 

Overhead  linework 68 

Poles    69 

Pole  guying 72 

Corners    75 

Cross  arms    75 

Service  mains 77 

Lightning  arresters   78 

Lamp  supports  on  poles 79 

Underground  linework 79 

Iron  pipe 80 

Vitrified  tile  conduit 80 

Laying  conduit 82 

Fibre  conduit  .  .  83 


PART  II 
ELECTRIC  LIGHTING 

PAGE 

History  and  development 1 

Classification 2 

Incandescent  lamps 2 

Manufacture  of  carbon  filament  lamps V 3 

Voltage  and  candle-power 5 

Efficiency    6 

Selection  of  lamps 8 

Distribution  curves   11 

Gem  metallized- filament  lamp 12 

Metallic  filament  lamps 14 

Helion  lamp   21 

Nernst  lamp 21 

Comparison  of  different  types 25 


CONTENTS 


PAGE 


Special  lamps  27 

Mercury  vapor  lamp 27 

Moore  tube  light 29 

Arc  lamps  32 

Control  mechanisms 33 

Types  of    38 

Direct-current  arcs 38 

Alternating-current  arcs   39 

Flaming  arcs 42 

Power  distribution  systems 46 

Illumination    '.,." 53 

Residence  lighting 56 

Types  of  lamps 56 

Illumination  calculations   .  .". 56 

Intensity  constants  for  incandescent  lamps.  . 59 

Intensity  constants  for  arc  lamps 59 

Arrangements  of  lamps. 55 

Lighting  of  public  halls,  offices,  etc 64 

Street  lighting   67 

Shades  and  reflectors 72 

Frosted  globes 73 

Holophane  globes 74 

Opal  enclosed  globes 76 

Photometry ,. 76 

Light  standards  .......;..  .  . . .- * 76 

Photometers    79 

Lummer-Brodhun 80 

Weber    83 

Portable 85 

Integrating  86 

Incandescent  lamp  photometry 88 

Arc  light  photometry 93 


INTRODUCTION 

T~^  LECTEIC  lighting  virtually  started  with  the  invention  of  the 
' — J  Edison  incandescent  lamp  in  1878,  the  discovery  of  this  very 
useful  and  flexible  lighting  unit  marking  an  epoch  not  only  in  home 
lighting.,  but  also  in  the  actual  development  of  the  electrical  industry 
itself.  This  invention  had  been  preceded  by  the  invention  of  the  higher 
powered  but  less  flexible  arc  lamp,  and  these  two  fundamental  lighting 
sources  have  been  the  standards  of  electrical  illumination  since  that 
time. 

C  The  last  few  years  have  seen  many  notable  improvements,,  not  only 
in  the  methods  of  lighting  but  in  the  light  units  themselves.  The 
enclosed  arc,  the  flaming  arc,  the  Moore  tubes,  and  the  Nernst  lamp 
have  all  contributed  to  this  wonderful  development,  but  the  recent 
improvement  in  metallic  filament  lamps,  notably  the  Tungsten,  has 
given  an  impetus  which  is  second  onty  to  the  original  invention  of  the 
incandescent  lamp  itself.  To  cut  the  energy  consumption  per  candle- 
power  from  3.5  watts  to  1.25  watts  is  certainly  a  triumph,  and  this 
improvement  has  opened  up  many  fields  of  activity  hitherto  closed  to 
incandescent  lighting.  It  has  even  made  the  beautifully  effective,  but 
criminally  inefficient,  method  of  indirect  lighting  economically  pos- 
sible. 

C  In  addition  to  the  progress  in  the  lighting  phases  of  this  interesting 
subject,  the  many  precautions  and  safeguards  which  the  building 
departments  of  our  cities  and  the  National  Board  of  Fire  Underwriters 
demand  in  connection  with  lighting  and  power  circuits,  make  it  all  the 


more  necessary  that  everyone  actively  engaged  or  interested  in  lighting 
and  wiring  should  have  a  reliable  handbook  giving  standard  specifica- 
tions and  requirements  as  to  materials  and  methods,  and  adequate 
descriptions  of  recommended  devices. 

C  The  material  in  this  volume  is  especially  adapted  for  purposes  of 
self-instruction  and  home  study.  The  utmost  care  has  been  used  to 
make  the  treatment  of  each  subject  appeal  not  only  to  the  technically 
trained  expert,  but  also  to  the  beginner  and  the  self-taught  practical 
man  who  wishes  to  keep  abreast  of  modern  progress.  The  language 
is  simple  and  clear;  heavy  technical  terms  and  the  formulae  of  the 
higher  mathematics  have  been  avoided,  yet  without  sacrificing  any  of 
the  requirements  of  practical  instruction. 

C  For  purposes  of  ready  reference  and  timely  information  when 
needed,  it  is  believed  that  this  handbook  will  be  found  to  meet  every 
requirement. 


ELECTRIC  WIRING 


METHODS  OF  WIRING 

The  different  methods  of  wiring  which  are  now  approved  by  the 
National  Board  of  Fire  Underwriters,  may  be  classified  under  four 
general  heads,  as  follows: 

1.  WIRES  RUN  CONCEALED  IN  CONDUITS. 

2.  WIRES  RUN  IN  MOULDING. 

3.  CONCEALED  KNOB  AND  TUBE  WIRING. 

4.  WIRES  RUN  EXPOSED  ON  INSULATORS. 

WIRES  RUN  CONCEALED  IN  CONDUITS 

Under  this  general  head,  will  be  included  the  following: 

(a)     Wires  run  in  rigid  conduits. 

(6)     Wires  run  in  flexible  metal  conduits. 

(c)     Armored  cable. 

Wires  Run  in  Rigid  Conduit.  The  form  of  rigid  metal  conduit  now 
used  almost  exclusively,  consists  of  plain  iron  gaspipe  the  interior  sur- 
face of  which  has  been  prepared  by  removing  the  scale  and  by  remov- 
ing the  irregularities,  and  which  is  then  coated  with  flexible  enamel. 
The  outside  of  the  pipe  is  given  a  thin  coat  of  enamel  in  some  cases, 
and,  in  other 
cases,  is  galvan- 
i z e d  .  Fig.  1 
shows  one  make 

»  11    ,  Fig.  1.    Rigid  Enameled  Conduit,  Unlined. 

OI  enameled  (nu-  Courtesy  of  American  Conduit  Mfg.  Co.,  Pittsburg,  Pa. 

lined)  conduit. 

Another  form  of  rigid  conduit  is  that  known  as  the  armored  con- 
duit, which  consists  of  iron  pipe  with  an  interior  lining  of  paper 
impregnated  with  asphaltum  or  similar  compound.  This  latter  form 
of  conduit  is  now  rapidly  going  out  of  use,  owing  to  the  unlined  pipe 
being  cheaper  and  easier  to  install,  and  owing  also  to  improved  methods 
of  protecting  the  iron  pipe  from  corrosion,  and  to  the  introduction  of 
additional  braid  on  the  conductors,  which  partly  compensates  for  the 


ELECTRIC  WIRING 


pipe  being  unlined.  The  introduction  of  improved  devices — such  as 
outlet  insulators,  for  protecting  the  conductors  from  the  sharp  edges  of 
the  pipe,  at  outlets,  cut-out  cabinets,  etc. — also  decreases  the  neces- 
sity of  the  additional  protection  afforded  by  the  interior  paper  lining. 

Rigid  Conduits  are  made  in  gaspipe  sizes,  from  one-half  inch  to 
three  inches  in  diameter.  The  following  table  gives  the  various  data 
relating  to  rigid,  enameled  (unlined)  conduit: 

TABLE  I 
Rigid,  Enameled  Conduit — Sizes,  Dimensions,  Etc. 


NOMINAL 

NUMBER  OF 

ACTUAL 

NOMINAL 

STANDARD 
PIPE  SIZE 

THICKNESS 

WEIGHT 

PER 

THREADS 
PER  INCH 

OUTSIDE 
DIAMETER. 

INSIDE 
DIAMETER. 

100  FEET 

OF  SCREW 

INCHES 

INCHES 

j 

.109 

84 

14 

.84 

.62 

| 

.113 

112 

14 

1.05 

.82 

1 

.134 

167 

11* 

1.31 

1.04 

H 

.140 

224 

11* 

1.66 

1.38 

i* 

.145 

268 

11* 

1.90 

1.61 

2 

.154 

361 

111 

2.37 

2.06 

2i 

.204 

574 

8 

2.87 

2.46 

3 

.217 

754 

8 

3.50 

3.06 

Tables  II,  III,  and  IV  give  the  various  sizes  of  conductors  that 
may  be  installed  in  these  conduits.     Caution  must  be  exercised  in 

TABLE  II 
Single  Wire  in  Conduit 


SIZE  WIRE,  B.  &  S.  G. 

LORICATED  CONDUIT,  UNLINED;  D.  B.  WIRE 

No.  14-4 

\  inch 

2 

1      ' 

1 

i    ' 

0 

f  inch  or  1         ' 

00 

1 

000 

1          ' 

0000 

1 

t 

250,000  C.  M. 

H 

( 

300,000  C.  M. 

H 

( 

350,000  C.  M. 

H 

i 

400,000  C.  M. 

H       "     or  if 

t 

450.000  C.  M. 

l| 

i 

SOOiOOO  C.  M. 

1* 

i 

600,000  C.  M. 

H       "     or  2 

1 

700,000  C.  M. 

2 

i 

800,000  C.  M. 

2 

* 

900,000  C.  M. 

2 

t 

1,000,000  C.  M. 

2        "     or  2£ 

i  . 

1,500,000  C.  M. 

2* 

t 

1,700,000  C.  M. 

( 

2,000,000  C.  M. 

3 

i 

ELECTRIC  WIRING 


TABLE   III 
Two  Wires  in  One  Conduit 


SIZE  WIRE,  B.  &  S.  G. 

LORICATED  CONDUIT,  UNLINED;   D.  B.  WIRE 

No.  14 

\  inch. 

12 

\  irch  or      f 

' 

10 

| 

< 

8 

1 

< 

6 

1 

' 

5 

1         "      or  \\ 

' 

4 

H 

t 

3 

H 

( 

2 

U      "     or  \\ 

1 

1 

lj 

1 

0 

H 

f 

00 

H      "      or  2 

' 

000 

2 

1 

0000 

2 

1 

250,000  C.  M. 

2         "      or  2i 

1 

300,000  C.  M. 

2i 

t 

350,000  C.  M. 

2^ 

i 

400,000  C.  M. 

2£      "     or  3 

i 

450,000  C.  M. 

3 

< 

500,000  C.  M. 

3 

: 

600,000  C.  M. 

3 

t 

700,000  C.  M. 

3 

( 

TABLE  IV 
Three  Wires  in  One  Conduit 


SIZE  WIRE,  B.  &  S.  G. 


LORICATED   TUBE,   UNLINED; 


Outside 

Center 

D.  B.  WIRE 

No. 

14 

No. 

12 

f  inch 

12 

10 

£     " 

10 

8 

1 

8 

6 

1 

6 

4 

1* 

5 

2 

H 

4 

1 

1|    inch  or  1^ 

3 

0 

1* 

2 

2/0 

li      "      or  2 

1 

3/0 

2 

0 

4/0 

2 

2/0 

250 

M. 

2        "      or  2J 

3/0 

300 

M. 

2* 

4/0 

400 

M. 

2* 

250 

M. 

450 

M. 

2J      "      or  3 

250 

M. 

500 

M. 

3 

300 

M. 

600 

M. 

3 

350 

M. 

700 

M. 

3 

400 

M. 

800 

M. 

3 

450 

M. 

900 

M. 

3 

4  ELECTRIC  WIRING 

using  these  tables,  for  the  reason  that  the  sizes  of  conductors  which 
may  be  safely  installed  in  any  run  of  conduit  depend,  of  course,  upon 
the  length  of  and  the  number  of  bends  in  the  run.  The  tables  are 
based  on  average  conditions  where  the  run  does  not  exceed  90  to  100 
feet,  without  more  than  three  or  four  bends,  in  the  case  of  the  smaller 
sizes  of  wires  for  a  given  size  of  conduit ;  and  where  the  run  does  not 
exceed  40  to  50  feet,  with  not  more  than  one  or  two  bends,  in  the  case 
of  the  larger  sizes  of  wires,  for  the  same  sizes  of  conduit. 

Unlined  conduit  can  be  bent  without  injury  to  the  conduit,  if  the 
conduit  is  properly  made  and  if  proper  means  are  used  in  making  the 
bends.  Care  should  be  exercised  to  avoid  flattening  the  tube  as  a  result 
of  making  the  bend  over  a  sharp  curve  or  angle. 

In  installing  iron  conduits,  the  conduits  should  cross  sleepers  or 
beams  at  right  angles,  so  as  to  reduce  the  amount  of  cutting  of  the 
beams  or  sleepers  to  a  minimum. 

Where  a  number  of  conduits  originate  at  a  center  of  distribution, 
they  should  be  run  at  right  angles  for  a  distance  of  two  or  three  feet 
from  the  cut-out  box,  in  order  to  obtain  a  symmetrical  and  workman- 
like arrangement  of  the  conduits,  and  so  as  to  have  them  enter  the 
cabinet  in  a  neat  manner.  While  it  is  usual  to  use  red  or  white  lead 
at  the  joints  of  conduits  in  order  to  make  them  water-tight,  this  is 
frequently  unnecessary  in  the  case  of  enameled  conduit,  as  there  is 
often  sufficient  enamel  on  the  thread  to  make  a  water-tight  joint. 

When  iron  conduits  are  installed  in  ash  concrete,  in  Keene 
cement,  or,  in  general,  where  they  are  subject  in  any  way  to  corrosive 
action,  they  should  be  coated  with  asphaltum  or  other  similar  protec- 
tive paint  to  prevent  such  action. 

While  the  cost  of  circuit  work  run  in  iron  conduits  is  usually 
greater  than  any  other  method  of  wiring,  it  is  the  most  permanent 
and  durable,  and  is  strongly  recommended  where  the  first  cost  is  not 
th'e  sole  consideration.  This  method  of  wiring  should  always  be 
used  in  fireproof  buildings,  and  also  in  the  better  class  of  frame  build- 
ings. It  is  also  to  be  recommended  for  exposed  work  where  the  work 
is  liable  to  disturbance  or  mechanical  damage. 

Wires  Run  in  Flexible  Metal  Conduit.  This  form  of  conduit, 
shown  in  Fig.  2,  is  described  by  the  manufacturers  as  a  conduit  com- 
posed of  "concave  and  convex  metal  strips  wound  spirally  upon  each 
other  in  such  a  manner  as  to  interlock  several  concave  surfaces  and 


ELECTRIC  WIRING 


Fig.  2.    Flexible  Steel  Conduit. 
Courtesy  of  Sterling  Electric  Co.,  Troy,  N.  7. 


present  their  convex  surfaces,  both  exterior  and  interior,  thereby 
securing  a  smooth  and  comparatively  frictionless  surface  inside  and 
out." 

The  field  for  the  use  of  this  form  of  conduit  is  rapidly  increasing. 
Owing  to  its  flexibility,  conduit  of  this  type  can  be  used  in  numerous 
cases  where  the 
rigid  conduit 
could  not  possi- 
bly  be  em- 
ployed. Its  use 
is  to  be  recom- 
mended above 

all  the  other  forms  of  wiring,  except  that  installed  in  rigid  conduits. 
For  new  fireproof  buildings,  it  is  not  so  durable  as  the  rigid  conduit, 
because  not  so  water-tight;  and  it  is  very  difficult,  if  not  impossible, 
to  obtain  as  workmanlike  a  condu't  system  with  the  flexible  as  with  the 
rigid  type  of  conduit.  For  completed  or  old  frame  buildings,  however, 
the  use  of  the  flexible  conduit  is  superior  to  all  other  forms  of  wiring. 

Table  V  gives  the  inside  diameter  of  various  sizes  of  flexible  con- 
duit, and  the  lengths  of  standard  coils.  inside  diameter  of  this 
conduit  is  the  same  as  that  of  the  rigid  conduit;  and  the  table  given 
for  the  maximum  sizes  of  conductors  which  may  be  installed  in  the 
various  sizes  of  conduits,  may  be  used  also  for  flexible  steel  conduits, 
except  that  a  little  more  margin  should  be  allowed  for  flexible  steel 
conduits  than  for  the  rigid  conduits,  as  the  stiffness  of  the  latter  makes 
it  possible  to  pull  in  slightly  larger  sized  conductors. 

TABLE  V 
Greenfield  Flexible  Steel  Conduit 


INSIDE  DIAMETER 


APPROXIMATE  FEET  IN  COIL 


ft  inch 


? 


inches 


200 
200 
100 

50 

50 

50 

50 
Random  Lengths 


6 


ELECTRIC  WIRING 


This  conduit  should,  of  course,  be  first  installed  without  the  con- 
ductors, in  the  same  manner  as  the  rigid  conduit.  Owing  to  the 
flexibility  of  this  conduit,  however,  it  is  absolutely  essential  to  fasten 
it  securely  at  all  elbows,  bends,  or  offsets;  for,  if  this  is  not  done,  con- 
siderable difficulty  will  be  ex- 
perienced in  drawing  the  con- 
ductors in  the  conduit. 

The  rules  governing  the  in- 
stallation of  this  conduit  are 
the   same    as    those  covering 
rigid  conduits.  Double-braided 
Fig.  3.  use  of  Elbow  ciamp  for  Fastening  Flex-  conductors  are  required,  and 

ible  Conduit  in  Place.  '  ,  .    .      _        _  .  / 

the  conduit  should  be  grounded 

as  required  by  the  Code  Rules.  As  already  stated,  the  conduit  should 
be  securely  fastened  (in  not  less  than  three  places)  at  all  elbows;  or 
else  the  special  elbow  clamp  made  for  this  purpose,  shown  in  Fig.  3, 
.should  be  used. 

In  order  to  cut  flexible  steel  conduit  properly,  a  fine  hack  saw 
should  be  employed.  Outlet -boxes  are  required  at  all  outlets,  as  well 
as  bushing  and  wires  to  rigid 
conduit.  Fig.  4  shows  a  coil 
of  flexible  steel  conduit.  Figs. 
5,  6,  and  7  show,  respectively, 
an  outlet  box  and  cover,  outlet 
plate,  and  bushing  used  for  this 
conduit. 

Armored  Cable.  There 
are  many  cases  where  it  is  im- 
possible to  install  a  conduit 
system.  In  such  cases,  prob- 
bably  the  next  best  results  may 
be  obtained  by  the  use  of  steel 
armored  cable.  The  rules  gov- 
erning the  installation  of  armored  cable  are  given  in  the  National 
Electric  Code,  under  Section  24-A,  and  Section  48 ;  also  in  24r-S.  This 
cable  is  shown  in  Fig.  8. 

Steel  armored  cable  is  made  by  winding  formed  steel  strips  over 
the  insulated  conductors.    The  steel  strips  are  similar  to  those  used 


Fig.  4.  A  100-Foot  Coil  of  Flexible  Steel  Conduit. 
Courtesy  of  Sprague  Electric  Co.,  New  York,N.Y. 


ELECTRIC  WIRING  7 

for  the  steel  conduit.     Care  is  taken  in  forming  the  cable,  to  avoid 
crushing  or  abraiding  the  insulation  on  the  conductors  as  the  steel 


Fig.  5.    Outlet  Box  for  Flexible  Steel  Conduit. 

strips  are  fed  and  formed  over  the  same.     In  the  process  of  manufac- 
ture, the  spools  of  steel  ribbon  are  of  irregular  length,  and  when  a 


Fig.  6.    Outlet  Plate  for  Flexible  Steel  Fig.  7.    Outlet  Bushing. 

Conduit.  Courtesy  of  SpragueElectric  Co.,NewYork,  2V.  F. 

spool  is  empty,  the  machine  is  stopped,  and  the  ribbon  is  started  on 
the  next  spool,  the  process  being  continued.    There  is  no  reason  why 


Fig.  8.    Flexible  Armored  Cable.    Twin  Conductors. 
Courtesy  of  Sprague  Electric  Co.,  New  York,  N.  Y. 

the  conduit  cables  could  not  be  made  of  any  length ;  but  their  actual 
lengths  as  made  are  determined  by  convenience  in  handling.    Armored 


8 


ELECTRIC  WIRING 


cable  is  made  in  single  conductors  from  No.  1  to  No.  10  B.  &  S.  G.; 
in  twin  conductors,  from  No.  6  to  No.  14  B.  &  S.  G.;  and  three-conduc- 
tor cable,  from  No.  10  to  No.  14  B.  &  S.  G.  Table  VI  gives  various 
data  relating  to  armored  conductors: 

TABLE  VI 
Armored  Conductors — Types,  Dimensions,  Etc. 


SIZE 
B.&S 

GAUGE 

TYPE  AND  NUMBER  OF  CONDUCTORS 

OUTSIDE 
DIAMETER 

(INCHES) 

No.  14 

BX  twin  conductor 

.63 

"     12 

it       ti             it 

.685 

"     10 

(t       (t             (t 

.725 

"      8 

ti       tt             it 

.875 

"      6 

<t       it             ti 

1.3125 

"     14 

BM  twin  conductor  (for  marine  work  —  ship  wiring) 

.725 

"     12 

a       it              u 

.725 

"     10 

tt       <t             tt 

.73 

"    14 

BX3  three  conductor 

.71 

"    12 

ti         it              tt 

.725 

tt    10 

ti         tt              tt 

.73 

"     14 

BXL  twin  conductor,  leaded 

.725 

"     12 

it        tt             ti               tt 

.725 

"     10 

tt        ti             tt               a 

.87 

"    14 

BXL3  three  conductor,  leaded 

.90 

"    12 

ti           tt             tt                ti 

.90 

"    10 

tt           n             tt                tt 

.94 

"    10 
"      8 

Type  D  single  conductor,  stranded 

.550 
.550 

"      6 

it                t 

.575 

"      4 

tt                i 

.700 

"      2 

tt                t 

.900 

"      1 

tt                t 

.965 

"    10 
11      8 

Type  DL  single  conductor,  stranded,  leaded 

if                tt           -i                                   (i 

.625 
.710 

"      6 

it                 ft             t                                   (t 

.700 

«      4 

tt                 ti             t                                   n 

.760 

"      2 

ti                tt                                                tt 

.920 

"      1 

u                K             t                                   it 

.910 

STEEL  ARMORED  FLEXIBLE  CORD 

"    18 
"    16 

Type  E  twin  conductor 

it      tt      ti             ti 

.40 
.40 

"     14 

ti      ft      ti             it 

.47 

"    18 
"    16 

Type  EM  twin  conductor,  re-inforced 

if      ti        tt            ti                   tt 

.575 

.585 

"    14 

<t        tt        tt            ti                   ti 

.595 

In  Table  VI,  Tvpes  D  (single),  BX  (twin),  and  BX3  (3  conduc- 


ELECTRIC  WIRING  9 

tors)  are  armored  cable  adapted  for  ordinary  indoor  work.  Type 
BM  (twin  conductors)  is  adapted  for  marine  wiring.  Types  DL 
(single),  BXL  (twin),  and  BXL  3  (3  conductors)  have  the  conductors 
lead -encased,  with  the  steel  armor  outside,  and  are  especially  adapted 
for  damp  places,  such  as  breweries,  stables,  and  similar  places. 

Type  E  is  used  for  flexible-cord  pendants,  and  is  suitable  for 
factories,  mills,  show  windows,  and  other  similar  places.  Type  EM 
is  the  same  as  Type  E;  but  the  flexible  cord  is  reinforced,  and  is  suit- 
able for  marine  work,'  for  use  in  damp  places,  and  in  all  cases  where  it 
will  be  subject  to  very  rough  handling. 

While  this  form  of  wiring  has  not  the  advantage  of  the  conduit 
system — namely,  that  the  wires  can  be  withdrawn  and  new  wires 
inserted  without  disturbing  the  building  in  any  way  whatever — yet  it 
has  many  of  the  advantages  of  the  flexible  steel  conduit,  and  it  has 
some  additional  advantages  of  its  own.  For  example,  in  a  building 
already  erected,  this  cable  can  be  fished  between  the  floors  and  in  the 
partition  walls,  where  it  would  be  impossible  to  install  either  rigid 
conduit  or  flexible  steel  conduit  without  disturbing  the  floors  or 
walls  to  an  extent  that  would  be  objectionable. 

Armored  conductors  should  be  continuous  from  outlet  to  outlet, 
without  being  spliced  and  installed  on  the  loop  system.  Outlet  boxes 
should  be  installed  at  all  outlets,  although,  where  this  is  impossible, 
outlet  plates  may  be  used  under  certain  conditions.  Clamps  should 
be  provided  at  all  outlets,  switch-boxes,  junction-boxes,  etc.,  to  hold 
the  cable  in  place,  and  also  to  serve  as  a  means  of  grounding  the  steel 
sheathing. 

Armored  cable  is  less  expensive  than  the  rigid  conduit  or  the 
flexible  steel  conduit,  but  more  expensive  than  cleat  wiring  or  knob 
and  tube  wiring,  and  is  strongly  recommended  in  preference  to  the 
latter. 

WIRES  RUN  IN  MOULDING 

Moulding  is  very  extensively  used  for  electric  circuit  work,  in 
extending  circuits  in  buildings  which  have  already  been  wired,  and 
also  in  wiring  buildings  which  were  not  provided  with  electric  circuit 
work  at  the  time  of  their  erection.  The  reason  for  the  popularity  of 
moulding  is  that  it  furnishes  a  convenient  and  fairly  good-looking 
runway  for  the  wires,  and  protects  them  from  mechanical  injury. 


10 


ELECTRIC  WIRING 


i 

y              T 

*a 

3 

IT 

d  co 

CQ  J_ 

Ac  k  Ab  4*  Aa4*-  Ab  *|  Ac 

A  « 

Fig.  9.    Two- Wire  Wood  Moulding. 


It  seems  almost  unwise  to  place  conductors  carrying  electric  current, 
in  wood  casing;  but  this  method  is  still  permitted  by  the  National 
Electric  Code,  although  it  is  not  allowed  in  damp  places  or  in  places 

where  there  is  liability  to  damp- 
ness, such  as  on  brick  walls, 
in  cellars,  etc. 

The  dangers  from  the  use  of 
moulding  are  that  if  the  wood 
becomes  soaked  with  water, 
there  will  be  a  liability  to  leak- 
age of  current  between  the  conductors  run  in  the  grooves  of  the  mould- 
ing, and  to  fire  being  thereby  started,  which  may  not  be  immediately  dis- 
covered. Furthermore,  if  the  conductors  are  overloaded,  and  conse- 
quently overheated,  the  wood  is  likely  to  become  charred  and  finally  ig- 
nited. Moreover,  the  moulding  itself  is  always  a  temptation  as  affording 
a  good  "round  strip"  in  which  to  drive  nails,  hooks,  etc.  However,  the 
convenience  and  popularity  of  moulding  cannot  be  denied;  and  until 
some  better  substitute  is  found,  or  until  its  use  is  forbidden  by  the 
Rules,  it  will  continue  to  be  used  to  a  very  great  extent  for  running 
circuits  outside  of  the  walls  and  on  the  ceilings  of  existing  build  ings. 
Figs.  9, 10, 11,  and  12  show  two- and  three-wire  moulding  respectively; 
and  Table  VII  gives  complete  data  as  to  sizes  of  the  moulding  required 
for  various  sizes  of  conductors. 

While  the  Rules  recommend  the  use  of  hardwood  moulding,  as  a 
matter  of  fact  probably  90  per  cent  of  the  moulding  used  is  of  white- 
wood  or  other  similar  cheap,  soft  wood .  Georgia  pine  or  oak  ordinarily 


Q 

^                                                                                     ^1 

t 

o 

00 

d   1 

Q    i 

—  Ac-» 

Ab 

-Aa- 

-Ab- 

*-Ac^ 

c 

i 

I 

Fig.  10.    Two-Wire  Wood  Moulding. 


costs  about  twice  as  much  as  the  soft  wood.  In  designing  moulding 
work,  if  appearance  is  of  importance,  the  moulding  circuits  should 
be  laid  put  so  as  to  afford  a  symmetrical  and  complete  design.  For 


ELECTRIC  WIRING 


11 


example,  if  an  outlet  is  to  be  located  in  the  center  of  the  ceiling, 
the  moulding  should  be  continued  from  wall  to  wall,  the  portion  beyond 
the  outlet,  of  course,  having  no  conductors  inside  of  the  moulding. 
If  four  outlets  are  to  be  placed  on  the  ceiling,  the  rectangle  of  moulding 
should  be  completed  on  the  fourth  side,  although,  of  course,  no  con- 


i 

z                 E 

I 

- 
Jl  ? 

Ac]*-Ab-j-Aa4-Ab  j-A 

aT~Ab-*mc 

.                     A 

Fig.  11.    Three- Wire  Wood  Moulding. 

ductors  need  be  placed  in  this  portion  of  the  moulding.  Doing  this 
increases  the  cost  but  little  and  adds  greatly  to  the  appearance. 

Moulding  is  frequently  used  in  combination  with  other  methods 
of  wiring,  including  armored  cable,  flexible  steel  tubing,  and  fibrous 
tubing.  In  many  instances,  it  is  possible  to  fish  tubing  between 
beams  or  studs  running  in  a  certain  direction;  but  when  the  conduc- 
tors are  to  run  in  another  direction  or  at  right  angles  to  the  beams  or 
studs,  exposed  work  is  necessary.  In  such  cases,  a  junction-box  or 
outlet-box  must  be  placed  at  the  point  of  connection  between  the 
moulding  and  the  armored  cable  or  steel  tubing. 

Where  circuits  are  run  in  moulding,  and  pass  through  the  floor, 
additional  protection  must  be  provided,  as  required  by  the  Code  Rules, 


Q 
D 

^                                                                                                                              x 

DC 

0 
DQ 

|J 

—Ac— 

—  Ab-* 

—  Act— 

—  Ab— 

—  ACL— 

—  Ab- 

-Ac- 

Q 

Fig.  12.    Three-Wire  Wood  Moulding. 

to  protect  the  moulding.  As  a  rule,  it  is  better  to  use  conduit  for  all 
portions  of  moulding  within  six  feet  of  the  floor,  so  as  to  avoid  the 
possibility  of  injury  to  the  circuits.  Where  a  combination  of  iron 
conduit  or  flexible  steel  tubing  is  used  with  moulding,  it  is  well  to  use 
double-braided  conductors  throughout,  because,  although  only  single- 


12 


ELECTRIC  WIRING 


TABLE  VII 
Sizes  of  Mouldings  Required  for  Various  Sizes  of  Conductors 


OF 
DING 


Y 

M 


A-2 


MBER 
WIRES 


MAXIMUM 
SIZE  OF  WIRE 
BANDS. 


SOLID 


12 


STRANDED 


14 


DIMENSIONS    IN  INCHES 


AaAb 


Ac 


5 


Ba 


Bb 


32 


DC 


Ca 


A-4 


6 


10 


_ 

16 


h 


29 
32 


16 


32 


'fl 


16 


A-6 


It 


A-6 


A-9 


10 


A-IO 


55QOOO 
C.M. 


16 


10 


A-l 


C.M. 


\i 


. 

16 


1  1 


B-2 


12 


14 


. 

32 


27 
32 


32 


i  1 


B-4- 


6 


10 


L5 
32 


. 

16 


16 


29 
32 


16 


32 


1  I 


B-6 


13 
32 


1  1 


B-8 


_ 
32 


16 


9_ 

32 


1  1 


B-9 


3/0 


16 


15 
32 


. 
32 


9. 
32 


12 


B-IO 


25QOOO 
C.M. 


23 
32 


23 
32 


. 
16 


12 


B-ll 


4-OQOOO 
C.M. 


16 


braided  conductors  are  required  with  moulding,  double-braided  con- 
ductors are  required  with  unlined  conduit,  and  if  double-braided  con- 
ductors were  not  used  throughout,  it  would  be  necessary  to  make  a 
joint  at  the  outlet-box  where  the  moulding  stopped  and  the  conduit 
work  commenced.  Where  the  conductors  pass  through  floors,  in 
moulding  work,  and  where  iron  conduit  is  used,  the  inspection  authori- 
ties, in  order  to  protect  the  wire,  usually  require  that  a  fibrous  tubing 
be  used  as  additional  protection  for  the  conductors  inside  of  the  iron 
pipe,  although,  if  double-braided  wire  is  used,  this  will  not  usually  be 
required.  Fig.  13  shows  a  f  useless  cord  rosette  for  use  with  moulding 
work.  Fig.  14  shows  a  device  for  making  a  tap  in  moulding  wiring. 

Moulding  work,  under  ordinary  conditions,  costs  about  one-half 
as  much  as  circuit  run  in  rigid  conduit,  and  about  75  per  cent,  under 


ELECTRIC  WIRING  13 

ordinary  conditions,  of  the  cost  of  armored  cable.  Where  the  latter 
method  of  wiring  or  the  conduit  system  can  be  employed,  one  or  the 
other  of  these  two  methods  should  be  used  in  preference  to  moulding, 


Fig.  13.     Fuseless  Cord  Fig.  14.    Device  for  Making  "Tap"  in 

Rosette.  Moulding. 

Courtesy  of  Grouse- TRnds  Co.,  Courtesy  of  II.  T.  Paiste  CD., 

Syracuse,  'N.  Y.  Philadelphia,  Pa. 

as  the  work  is  not  only  more  substantial,  but  also  safer.  Various  forms 
of  metal  moulding  have  been  introduced,  but  up  to  the  present  time 
have  not  met  with  the  success  which  they  deserve. 

CONCEALED  KNOB  AND  TUBE  WIRING 

This  method  of  wiring  is  still  allowed  by  the  National  Electric 
Code,  although  many  vigorous  attempts  have  been  made  to  have  it 
abolished.  Each  of  these  attempts  has  met  with  the  strongest 
opposition  from  contractors  and  central  stations,  particularly  in  small 
towns  and  villages,  the  argument  for  this  method  being,  that  it  is  the 
cheapest  method  of  wiring,  and  that  if  it  were  forbidden,  many  places 
which  are  wired  according  to  this  method  would  not  be  wired  at  all, 
and  the  use  of  electricity  would  therefore  be  much  restricted,  if  not 
entirely  done  away  with,  in  such  communities.  This  argument,  how- 
ever, is  only  a  temporary  makeshift  obstruction  in  the  way  of  inevitable 
progress,  and  in  a  few  years,  undoubtedly,  the  concealed  knob  and 
tube  method  will  be  forbidden  by  the  National  Electric  Code. 

The  cost  of  wiring  according  to  this  method  is  about  one-third 
of  the  cost  of  circuits  run  in  rigid  conduit,  and  about  one-half  of  the 
cost  of  circuits  run  in  armored  cable.  The  latter  method  of  wiring 


14 


ELECTRIC  WIRING 


is  rapidly  replacing  knob  and  tube  wiring,  and  justly  so,  wherever 
the  additional  price  for  the  latter  method  of  wiring  can  be  obtained. 
As  the  name  indicates,  this  method  of  wiring  employs  porcelain  knobs 


Fig.  15.    Knob  and  Tube  Wiring. 

and  tubes,  the  circuit  work  being  run  concealed  between  the  floor  beams 
and  studs  of  a  frame  building.  The  knobs  are  used  when  the  circuits 
run  parallel  to  the  floor  beams ;  and  the  porcelain  tubes  are  used  when 
the  circuits  are  run  at  right  angles  to  the  floor  beams. 

Fig.  15  shows  an  example  of  knob  and  tube  wiring.  In  concealed 
knob  and  tube  wiring,  the  wires  must  be  separated  at  least  five  inches 
from  one  another,  and  at  least  one  inch  from  the  surface  wired  over, 
that  is,  from  the  beams,  flooring,  etc.,  to  which  the  insulator  is  fas- 
tened. Fig.  16  shows  a 
good  type  of  porcelain 
knob  for  this  class  of 
wiring.  For  knob  and 
tube  wiring,  it  will  be 
noted  that,  owing  to  the 
fact  that  the  wiring  is 
concealed,  the  conductors  Fig" 1G  Porcelain  Knob' 

must  be  kept  further  apart  than  in  the  case  of  exposed  or  open  wiring 
on  insulators,  where,  except  in  damp  places,  the  wires  may  be  run  on 
cleats  or  on  insulators  only  one-half  inch  from  the  surface  wired  ovei, 


ELECTRIC  WIRING 


15 


Fibrous  Tubing.  Fibrous  tubing  is  frequently  used  with  knob 
and  tube  wiring,  and  the  regulations  governing  its  use  are  given  in 
Rule  24,  Section  S,  of  the  National  Electric  Code.  This  tubing,  as 
stated  in  this  Rule,  may  be  used  where  it  is  impossible  and  impracticable 
to  employ  knobs  and  tubes,  provided  the  difference  in  potential 
between  the  wires  is  not  over  300  volts,  and  if  the  wires  are  not  sub- 


Fig.  17.    Flexible  Tubing,  "Flexduct"  Type. 
Courtesy  of  National  Metal  Molding  Co.,  Pittsburg,  Pa. 


ject  to  moisture.  The  cost  of  wiring  in  flexible  fibrous  tubing  is 
approximately  about  the  same  as  the  cost  of  knob  and  tube  wiring. 
Duplex  conductors,  or  two  wires  together  are  not  allowed  in  fibrous 
tubing. 

Fibrous  tubing  is  required  at  all  outlets  where  conduit  or  armored 
cable  is  not  used  (as  in  knob  and  tube  wiring) ;  and,  as  required  by  the 
Rules,  it  must  extend  back  from  the  last  porcelain  support  to  one  inch 
beyond  the  outlet.  Fig.  17  shows  one  make  of  fibrous  tubing. 

Table  VIII  gives  the  maximum  sizes  of  conductors  (double- 
braided)  which  may  be  installed  in  fibrous  conduit. 


TABLE  VIII 
Sizes  of  Conductors  in  Fibrous  Conduit 


OUTSIDE   DIAMETER 

INSIDE  DIAMETER 

ONE  WIRE  IN 

TUBE 

\l  inch 

i  inch 

No.  12 

11 

1 

"      8 

If 

^ 

"      6 

¥ 

f 

"       1 

£ 

"      2/0 

ii^ 

1 

250,000 

C.  M. 

i& 

H 

400,000 

C.  M. 

ill 

1* 

750,000 

C.  M. 

If 

1,000,000 

C.  M. 

21 

2 

1,500,000 

C.  M. 

2| 

2i    ' 

2,000,000 

C.  M. 

16  ELECTRIC  WIRING 

WIRES  RUN  EXPOSED  ON  INSULATORS 

This  method  of  wiring  has  the  advantages  of  cheapness, durability, 
and  accessibility. 

Cheapness.  The  relative  cost  of  this  method  of  wiring  as  com- 
pared with  that  of  the  concealed  conduit  system,  is  about  fifty  per  cent 
of  the  latter  if  rubber-covered  conductors  are  used,  and  about  forty 
per  cent  of  the  latter  if  weatherproof  slow-burning  conductors  are  used. 
As  the  Rules  of  the  Fire  Underwriters  allow  the  use  of  weatherproof 
slow-burning  conductors  in  dry  places,  considerable  saving  may  be 
effected  by  this  method  of  wiring,  provided  there  is  no  objection  to  it 


Fig.  I&    Large  Feeders  Run  Exposed  on  Insulators. 

from  the  standpoint  of  appearance,  and  also  provided  that  it  is  not 
liable  to  mechanical  injury  or  disarrangement. 

Durability.  It  Is  a  well-known  fact  that  rubber  insulation  has  a 
relatively  short  life.  Inasmuch  as  in  this  method  of  wiring,  the  insula- 
tion does  not  depend  upon  the  insulation  of  the  conductors,  but  on 
the  insulators  themselves,  which  are  of  glass  or  porcelain,  this  system 
is  much  more  desirable  than  any  of  the  other  methods.  Of  course, 
if  the  conductors  are  mechanically  injured,  or  the  insulators  broken, 
the  insulation  of  the  system  is  reduced ;  but  there  is  no  gradual  dete- 
rioration as  there  is  in  the  c^se  of  other  methods  of  wiring,  where 


ELECTRIC  WIRING 


17 


rubber  is  depended  upon  for  insulation.  This  is  especially  true  in  hot 
places,  particularly  where  the  temperature  is  120°  F.  or  above.  For 
such  cases,  the  weatherproof  slow-burning  conductors  on  porcelain 
or  glass  insulators  are  especially  recommended. 

Accessibility.    The  conductors  being  run  exposed,  they  may  be 
readily  repaired  or  removed,  or  connections  may  be  made  to  the  same. 

This  method  of 
wiring  is  especially 
recommended  for 
mills,  factories,  and 
for  large  or  long 
feeder  conductors. 
Fig.  18  shows  ex- 
amples of  exposed 
large  feeder  con- 
ductors, installed  in  the  New  York  Life  Insurance  Building,  New 
York  City.  For  small  conductors,  up  to  say  No.  6  B.  &  S. 
G-auge  each,  porcelain  cleats  may  be  used  to  support  one,  two, 
or  three  conductors,  provided  the  distance  between  the  conduc- 


Fig.  19.    Two- Wire  Cleat. 


Fig.  20.    One- Wire  Cleat. 


Fig.  21.      Porcelain  Insulator   for 
Large  Conductors. 


tors  is  at  least  2J  inches  in  a  two-wire  system,  and  2J  inches 
between  the  two  outside  conductors  in  a  three-wire  system  where  the 
potential  between  the  outside  conductors  is  not  over  300  volts.  The 
cleat  must  hold  the  wire  at  least  one-half  inch  from  the  surface  to  which 
the  cleat  is  fastened;  and  in  damp  places  the  wire  must  be  held  at 
least  one  inch  from  the  surface  wired  over.  For  larger  conductors, 


18 


ELECTRIC  WIRING 


from  No.  6  to  No.  4/  OB.  &  S.  Gauge,  it  is  usual  to  use  single  porcelain 
cleats   or  knobs.     Figs.  19  and  20  show  a  good  form  of  two-wire 


Fig.  22.    Iron  Rack  and  Insulators  for  Large  Conductors. 
Courtesy  of  General  Electric  Co.,  Schenectady,  N.  Y. 


cleat  and  single-wire  cleat,  respectively. 

For  large  feeder  or  main  conductors 
from  No.  4/0  B.  &  S.  Gauge  upward,  a 
more  substantial  form  of  porcelain  insu- 
lafoi  should  be  used,  such  as  shown  in 
Fig.  21.  These  insulators  are  held  in 
iron  racks  or  angle-iron  frames,  of  which 
two  forms  are  shown  in  Figs.  22  and  23. 
The  latter  form  of  rack  is  particularly  de- 
sirable for  heavy  conductors  and  where  a 
number  of  conductors  are  run  together. 
In  this  form  of  rack,  any  length  of  con- 
ductor can  be  removed  without  disturb- 
ing the  other  conductors. 

As  a  rule,  the  porcelain  insulators 
should  be  placed  not  more  than  4J  feet 
apart;  and  if  the  wires  are  liable  tc  be 
disturbed,  the  distance  between  supports 
should  be  shortened,  particularly  for  small 
conductors.  If  the  beams  are  so  far 
apart  that  supports  cannot  be  obtained 
every  4J  feet,  it  is  necessary  to  provide  a 
running  board  as  shown  in  Fig.  24,  to 
which  the  porcelain  cleats  and  knobs 
can  be  fastened.  Figs.  25  and  26  show 
two  methods  of  supporting  small  con- 
ductors, For  conductors  of  No.  8  B.  &  S. 


Fig.  23.    Elevation  and  Plan  of 
Insulators  Held  in  Angle- 
Iron  Frames. 


ELECTRIC  WIRING 


19 


Gauge,  or  over,  it  is  not  necessary  to  break  around  the  beams,  provided 
they  are  not  liable  to  be  disturbed  ;  but  the  supports  may  be  placed  on 
each  beam. 

Where  the  dis- 
tance between  the 
supports,  however, 
is  greater  than  4J 
feet,  it  is  usually 
necessary  to  provide 

intermediate       SUp- 

ports,  as  shown  in 

Fig.  27,  or  else  to  provide  a  running-board.     Another  method  which 

may  be  used,   where  beams  are   further  than  4J  feet   apart,  is  to 


pigt24.    insulators  MOUK  ted  on  Running-Board  across  Wide- 

Spacec 


Fig.  25.    Method  of  Supporting  Small  Conductors. 


(B 

H 

\\f 

H 

_N-      .._                 ,      n 

-i_            _      y.    . 

Fig.  27.    Intermediate  Support  for  Conductor  between  Wide-Spaced  Beams. 

run  a  main  along  the  wall  at  right  angles  to  the  beams,  and  to 
have  the  individual  circuits  run  between  and  parallel  to  the  beams. 


nm 

y 

§" 

y. 

•»*-*— 

Fig.  26.    Method  of  Supporting  a  Small 
Conductor. 


Fig.  28.    Conductors  Protected  by  Wooden 
Guard-Strips  on  Low  Ceiling. 


In  low-ceiling  rooms,  where  the  conductors  are  liable  to  injury, 
it  is  usually  required  that  a  wooden  guard  strip  be  placed  on  each  side 
of  the  conductors,  as  shown  in  Fig.  28. 

Where  the  conductors  pass  through  partitions  or  walls,  they  must 


•20  ELECTRIC  WIRING 

be  protected  by  porcelain  tubes,  or,  if  the  conductors  be  of  rubber,  by 
means  of  fibrous  tubing  placed  inside  of  iron  conduits. 

All  conductors  on  the  walls  for  a  height  of  not  less  than  six  feet 
from  the  ground,  either  should  be  boxed  in,  or,  if  they  be  rubber-covered, 
should  (preferably)  be  run  in  iron  conduits;  and  in  conductors  having 
single  braid  only,  additional  protection  should  be  provided  by  means  of 
flexible  tubing  placed  inside  of  the  iron  conduit. 

Where  conductors  cross  each  other,  or  where  they  cross  iron  pipes, 
they  should  be  protected  by  means  of  porcelain  tubes  fastened  with 
tape  or  in  some  other  substantial  manner  that,  will  prevent  the  tubes 
from  slipping  out  of  place. 

TWO=WIRE  AND  THREE=WIRE  SYSTEMS 

As  both  the  two-wire  and  the  three-wire  system  are  extensively 
used  in  electric  wiring,  it  will  be  well  to  give  some  consideration  to  the 
advantages  and  disadvantages  of  each  system,  and  to  explain  them 
somewhat  in  detail. 

Relative  Advantages.  The  choice  of  either  a  two-wire  or  a  three- 
wire  system  depends  largely  upon  the  source  of  supply.  If,  for  ex- 
ample,  the  source  of  supply  will  always  probably  be  a  120-volt,  two 
wire  system,  there  would  be  no  object  in  installing  a  three-wire  system 
for  the  wiring.  If,  on  the  other  hand,  the  source  of  supply  is  a  120- 
240-volt  system,  the  wiring  should,  of  course,  be  made  three-wire. 
Furthermore,  if  at  the  outset  the  supply  were  two-wire,  but  with  a  pos- 
sibility of  a  three-wire  system  being  provided  later,  it  would  be  well 
to  adapt  the  electric  wiring  for  the  three-wire  system,  making  the 
neutral  conductor  twice  as  large  as  either  of  the  outside  conductors, 
and  combining  the  two  outside  conductors  to  make  a  single  conductor 
until  such  time  as  the  three-wire  service  is  installed.  Of  course,  there 
would  be  no  saving  of  copper  in  this  last-mentioned  three-wire  system, 
and  in  fact  it  would  be  slightly  more  expensive  than  a  two-wire  system, 
as  will  be  shortly  explained. 

The  object  of  the  three-wire  system  is  to  reduce  the  amount  of 
copper — and  consequently  the  cost  of  wiring — necessary  to  transmit  a 
given  amount  of  electric  power.  As  a  rule,  the  proposition  is  usually 
one  of  lighting  and  not  of  power,  for  the  reason  that  by  means  of  the 
three-wire  system  we  are  able  to  increase  the  potential  at  which  the 
current  is  transmitted,  and  at  the  same  time  to  take  advantage  of  the 


FT? 


ELECTRIC  WIRING  21 

greater  efficiency  of  the  lower  voltage  lamp.  If  current  for  power 
(motors,  etc.)  only  were  to  be  transmitted,  it  would  be  a  simple  matter 
to  wind  the  motors,  etc.,  for  a  higher  voltage,  and  thereby  reduce  the 
weight  of  copper. 
If,  however,  we  in- 
crease the  voltage 
of  lamps,  we  find 
that  they  are  not  so 
efficient,  nor  is  their 

life  SO   long.      With    F*&'  39«    Three-Wire  System,  with  Neutral  Conductor  between 

the  Two  Outside  Conductors. 

the  standard  carbon 

lamp,  it  has  been  found  that  the  240-volt  lamp,  with  the  same 
life,  requires  about  10  to  12  per  cent  more  current  than  the  cor- 
responding 120-volt  lamp.  Furthermore,  in  the  case  of  the  more 
efficient  lamps  recently  introduced  (such  as  the  Tantalum  lamp, 
Tungsten  lamp,  etc.),  it  has  been  found  impracticable,  if  not  impos- 
sible, to  make  them  for  pressures  above  125  volts.  For  this  reason 
the  three-wire  system  is  employed,  for  by  this  method  we  can  use  240 
volts  across  the  outside  conductors,  and  by  the  use  of  a  neutral  con- 
ductor obtain  120  volts  between  the  neutral  and  the  outside  conductor, 
and  thereby  be  enabled  to  use  120-volt  lamps.  Furthermore,  if  a 
240-volt  lamp  should  ever  be  placed  on  the  market  that  was  as  economi- 
cal as  the  lower  voltage  lamp,  the  result  would  be  that  the  240-480- 
volt  system  would  be  introduced,  and  240-volt  lamps  used.  As  a 

-.  matter  of  fact,  this 
has  been  tried  in 
several  cities — and 
particularly  in 
Providence,  Rhode 
^  Island.  As  a  rule, 

Fig.  30.    Lamps  Arranged  in  Pairs  in  Series,  Dispensing  with      however     the     1  20- 

Necessity  for  Third  or  Neutral  Conductor. 

volt  lamp  has  been 

found  so  much  more  satisfactory  as  regards  life,  efficiency,  etc.,  that 
it  is  nearly  always  employed. 

The  two-wire  system  is  so  extremely  simple  that  no  explanation 
whatever  is  required  concerning  it. 

The  three-wire  system,  however,  is  somewhat  confusing,  ana 
will  now  be  considered. 


22  ELECTRIC  WIRING 

Details  of  Three-Wire  System.  The  three-wire  system  may  be 
considered  as  a  two-wire  system  with  a  third  or  neutral  conductor 
placed  between  the  two  outside  conductors,  as  shown  in  Fig.  29. 
This  neutral  conductor  would  not  be  required  if  we  could  always  have 
the  lamps  arranged  in  pairs,  as  shown  in  Fig.  30.  In  this  case,  the 
two  lamps  would  burn  in  series,  and  we  could  transmit  the  current 
at  double  the  usual  voltage,  and  thereby  supply  twice  the  number  of 
lamps  with  one-quarter  the  weight  of  copper,  allowing  the  same  loss 
in  pressure  in  the  lamps.  The  reason  for  this  is,  that,  having  the 
lamps  arranged  in  series  of  pairs,  we  reduce  the  current  to  one-half, 
and,  as  the  pressure  at  which  the  current  Is  transmitted  is  doubled, 
we  can  again  reduce  the  copper  one-half  without  increasing  the  loss 
in  lamps.  We  therefore  see  that  we  have  a  double  saving,  as  the  cur- 
rent is  reduced  one-half,  which  reduces  the  weight  of  copper  one-half, 
and  we  can  again  reduce  the  copper  one-half  by  doubling  the  loss  in 
volts  without  increasing  the  percentage  loss.  For  example,'  if  in  one 
case  we  had  a  straight  two-wire  system  transmitting  current  to  100 
lamps  at  a  potential  of  100  volts,  and  this  system  were  replaced  by  one 
in  which  the  lamps  were  placed  in  series  of  pairs,  as  shown  in  Fig.  30, 
and  the  potential  increased  to  200  volts — 100  lamps  still  being  used — • 
we  should  find,  in  the  latter  case,  that  we  were  carrying  current  really 
for  only  50  lamps,  as  we  would  require  only  the  same  amount  of  cur- 
rent for  two  lamps  now  that  we  required  for  one  lamp  before.  Fur- 
thermore, as  the  potential  would  now  be  200  instead  of  100  volts, 
we  could  allow  twice  as  much  loss  as  in  the  first  case,  because  the  loss 
would  now  be  figured  as  a  percentage  of  200  volts  instead  of  a  percent- 
age of  100  volts.  From  this,  it  will  readily  be  seen  that  in  the  second 
case  mentioned,  we  would  require  only  one-quarter  the  weight  of 
copper  that  would  be  required  in  the  first  case. 

It  will  readily  be  seen,  however,  fchat  a  system  such  as  that  out- 
lined in  the  second  scheme  having  two  lamps,  would  be  impracticable 
for  ordinary  purposes,  for  the  reason  that  it  would  always  require  the 
lamps  to  be  burned  in  pairs.  Now,  it  is  for  this  very  reason  that  the 
third  or  neutral  conductor  is  required ;  and,  if  this  conductor  be  added, 
it  will  no  longer  be  necessary  to  burn  the  lamps  in  pairs.  This,  then, 
is  the  object  of  the  three-wire  system — to  enable  us  to  reduce  the 
amount  of  copper  required  for  transmitting  current,  without  increasing 
the  electric  pressure  employed  for  the  lamps. 


w 

g* 

« 

on 


ELECTRIC  WIRING  23 

With  regard  to  the  size  of  the  neutral  conductor,  one  important 
point  must  be  borne  in  mind ;  and  that  is,  that  the  Rules  of  the  National 
Electric  Code  require  the  neutral  conductor  in  all  interior  wiring  to  be 
made  at  least  as  large  as  either  of  the  two  outside  conductors.  The 
reasons  for  this  from  a  fire  standpoint  are  obvious,  because,  if  for 
any  reason  either  of  the  outside  conductors  became  disconnected,  the 
neutral  wire  might  be  required  to  carry  the  same  current  as  the  out- 
side conductors,  and  therefore  it  should  be  of  the  same  capacity.  Of 
course,  the  chances  of  such  an  event  happening  are  slight;  but,  as 
the  fire  hazard  is  all-important,  this  rule  must  be  complied  with  for 
interior  wiring  or  in  all  cases  where  there  would  be  a  probability  of 
fire.  For  outside  or  underground  work,  however,  where  the  fire 
hazard  would  be  relatively  unimportant,  the  neutral  conductor  might 
be  reduced  in  size;  and,  as  a  matter  of  fact,  it  is  made  smaller  than 
the  outside  conductors. 

The  three-wire  system  is  sometimes  installed  where  it  is  desired 
to  use  the  system  as  a  two-wire,  125-volt  system,  or  to  have  it  arranged 
so  that  it  may  be  used  at  any  time  also  as  a  three-wire,  125-250-volt 
system.  Of  course,  in  order  to  do  this,  it  is  necessary  to  make  the 
neutral  conductor  equal  to  the  combined  capacity  of  the  outside  con- 
ductors, the  latter  being  then  connected  together  to  form  one  con- 
ductor, the  neutral  being  the  return  conductor.  This  system  is  not 
recommended  except  in  such  instances,  for  example,  as  where  an 
isolated  plant  of  125  volts  is  installed,  and  where  there  is  a  possibility 
of  changing  over  at  some  future  time  to  the  three-wire,  125-250-volt 
system.  In  such  a  case  as  this,  however,  it  would  be  better,  where 
possible,  to  design  the  isolated  plant  for  a  three-wire,  125-250-volt 
system  originally,  and  then  to  make  the  neutral  conductor  the  same 
size  as  each  of  the  two  outside  conductors. 

The  weight  of  copper  required  in  a  three-wire  system  where  the 
neutral  conductor  is  the  same  size  as  either  of  the  two  outside  conduct- 
ors, is  f  of  that  required  for  a  corresponding  two-wire  system  using 
the  same  voltage  of  lamps.*  It  is  obvious  that  this  is  true,  because, 

*NOTE. — If,  in  the  two-wire  system,  we  represent  the  weight  of  each  of  the  two  con- 
ductors by  i,  the  weight  of  each  of  the  outside  conductors  in  a  three-wire  system  would 
be  represented  by  £;  and  if  we  had  three  conductors  of  the  same  size,  we  would  have 
i  +  i  +  i^fof  the  weight  of  copper  required  in  a  three-wire  system,  which  would  be 
required  in  a  corresponding  two-wire  system  having  the  same  percentage  of  loss  and 
using  the  same  voltage  of  lamps. 

If  the  neutral  conductor  were  made  \  of  the  size  of  each  of  the  outside  conductors, 
as  is  sometimes  done  in  underground  work,  the  total  weight  of  copper  required  would  be 
i  ^  i  f  T'S  =  A  of  that  required  in  the  corresponding  two- wire  system. 


24  ELECTRIC  WIRING 

as  the  discussion  proved  concerning  the  arrangement  shown  in  Fig. 
30,  where  the  lamps  were  placed  in  series  of  pairs,  we  found  that  the 
weight  of  copper  for  the  two  conductors  was  one-quarter  the  weight 
of  the  regular  two-wire  system.  It  is  then  of  course  true,  that,  if  we 
had  another  conductor  of  the  same  size  as  each  of  the  outside  conduct- 
ors, we  increase  theweight  of  copper  one-half,  or  one-quarter  plus 
one-half  of  one-quarter — that  is,  three-eighths. 

In  the  three-wire  system  frequently  used  in  isolated  plants  in 
which  the  two  outside  conductors  are  joined  together  and  the  neutral 
conductor  made  equal  to  their  combined  capacity,  there  is  no  saving 
of  copper,  for  the  reason  that  the  same  voltage  of  transmission  is  used, 
and,  consequently,  we  have  neither  reduced  the  current  nor  increased 
the  potential.  Furthermore,  though  the  weight  of  copper  is  the  same, 
it  is  now  divided  into  three  conductors,  instead  of  two,  and  naturally 
it  costs  relatively  more  to  insulate  and  manufacture  three  conductors 
than  to  insulate  and  manufacture  two  conductors  having  the  same 
total  weight  of  copper.  As  a  matter  of  fact,  the  three-wire  system, 
having  the  neutral  conductor  equal  to  the  combined  capacity  of  the 
two  outside  ones,  the  latter  being  joined  together,  is  about  8  to  10 
per  cent  more  expensive  than  the  corresponding  straight  two-wire 
system. 

In  interior  wiring,  as  a  rule,  where  the  three-wire  system  is  used 
for  the  mains  and  feeders,  the  two-wire  system  is  nearly  always  em- 
ployed for  the  branch  circuits.  Of  course,  the  two-wire  branch  cir- 
cuits are  then  balanced  on  each  side  of  the  three-wire  system,  so  as  to 
obtain  as  far  as  possible  at  all  times  an  equal  balance  on  the  two  sides 
of  the  system.  This  is  done  so  as  to  have  the  neutral  conductor  carry 
as  little  current  as  possible.  From  what  has  already  been  said,  it  is 
obvious  that  in  case  there  is  a  perfect  balance,  the  lamps  are  virtually 
in  series  of  pairs,  and  the  neutral  conductor  does  not  carry  any  current. 
Where  there  is  an  unbalanced  condition,  the  neutral  conductor  carries 
the  difference  between  the  current  on  one  side  and  the  current  on  the 
other  side  of  the  system.  For  example,  if  we  had  five  lamps  on  one 
side  of  the  system  and  ten  lamps  on  the  other,  the  neutral  conductor 
would  carry  the  current  corresponding  to  five  lamps. 

In  calculating  the  three-wire  system,  the  neutral  conductor  is 
disregarded,  the  outer  wires  being  treated  as  a  two-wire  circuit,  and 
the  calculation  is  for  one-half  the  total  number  of  lamps,  the  per- 


ELECTRIC  WIRING  25 

centage  of  loss  being  based  on  the  potential  across  the  two  outside 

conductors. 

• 

The  three-wire  system  is  very  generally  employed  in  alternating- 
current  secondary  wiring,  as  nearly  all  transformers  are  built  with 
three-wire  connections. 

While  unbalancing  will  not  affect  the  total  loss  in  the  outside 
conductors,  yet  it  does  affect  the  loss  in  the  lamps,  for  the  reason  that 
the  system  is  usually  calculated  on  the  basis  of  a  perfect  balance,  and 
the  loss  is  divided  equally  between  the  two  lamps  (the  latter  being 
considered  in  series  of  pairs).  If,  however,  there  is  unbalancing  to 
a  great  degree,  the  loss  in  lamps  will  be  increased ;  and  if  the  entire 
load  is  thrown  over  on  one  side,  the  loss  in  the  lamps  will  be  doubled 
on  the  remaining  side,  because  the  total  loss  in  voltage  will  now  occur 
in  these  lamps,  whereas,  in  the  case  of  perfect  balance,  it  would  be 
equally  divided  between  the  two  groups  of  lamps. 

CALCULATION  OF  SIZES  OF  CONDUCTORS 

The  formula  for  calculating  the  sizes  of  conductors  for  direct 
currents,  where  the  length,  load,  and  loss  in  volts  are  given,  is  as  fol- 
lows: 

The  size  of  conductor  (in  circular  mils)  is  equal  to  the  current  multiplied 
by  the  distance  (one  way),  multiplied  by  21.6,  divided  by  the  loss  in  volts;  or, 

CM  =  C  X  °FX  21-6 (I) 

in  which  C  =  Current,  in  amperes; 

D  —  Distance  or  length  of  the  circuit  (one  way,  in  feet)  ; 

V  =  Loss  in  volts  between  the  beginning  and  end  of  the  circuit. 

The  constant  (21.6)  of  this  formula  is  derived  from  the  resistance 
of  a  mil  foot  of  wire  of  98  per  cent  conductivity  at  25°  Centigrade  or 
77°  Fahrenheit.  The  resistance  of  a  conductor  of  one  mil  diam- 
eter and  one  foot  long,  is  10.8  at  the  temperature  and  conduc- 
tivity named.  We  multiply  this  figure  (10.8)  by  2,  as  the  length  of  a 
circuit  is  usually  given  as  the  distance  one  way,  and  in  order  to  obtain 
the  resistance  of  both  conductors  in  a  two-wire  circuit,  we  must 
multiply  by  2.  The  formula  as  above  given,  therefore,  is  for  a  two- 
wire  circuit;  and  in  calculating  the  size  of  conductors  in  a  three-wire 
system,  the  calculation  should  be  made  on  a  two- wire  basis,  as  ex- 
plained hereinafter. 


26  ELECTRIC  WIRING 

Formula  1  can  be  transformed  so  as  to  obtain  the  loss  in  a  given 
circuit,  or  the  current  which  may  be  carried  a  given  distance  with  a 
stated  loss,  or  to  obtain  the  distance  when  the  other  factors  are  given, 
in  the  following  manner: 

Formula  for  Calculating  Loss  in  Circuit  when  Size,  Current,  and  Distance  are  Giver, 
77  _    C  X  DX  21.6  f^^ 

CM  ......  (*) 

Formula  for  Calculating  Current  which  may  be  Carried  by  a  Given  Circuit  of  Specified 
Length,  and  with  a  Specified  Loss 

CMXV 


D  X  21.6 


Formula  for  Calculating  Length  of  Circuit  when  Size,  Loss,  and  Current  to  be  Carried 

are  Given 

CM  X  V  /A\ 

D=  ex  21.6 (4) 

Formulae  are  frequently  given  for  calculating  sizes  of  conductors, 
etc.,  where  the  load,  instead  of  being  given  in  amperes,  is  stated  in 
lamps  or  in  horse-power.  It  is  usually  advisable,  however,  to  reduce 
the  load  to  amperes,  as  the  efficiency  of  lamps  and  motors  is  a  variable 
quantity,  and  the  current  varies  correspondingly. 

It  is  sometimes  convenient,  however,  to  make  the  calculation 
in  terms  of  watts.  It  will  readily  be  seen  that  we  can  obtain  a  formula 
expressed  in  watts  from  Formula  1.  To  do  this,  it  is  advisable  to 
express  the  loss  in  volts  in  percentage,  instead  of  actual  volts  lost.  It 
must  be  remembered  that,  in  the  above  formulae,  V  represents  the 
volts  lost  in  the  circuit,  or,  in  other  words,  the  difference  in  potential 
between  the  beginning  and  the  end  of  the  circuit,  and  is  not  the 
applied  E.  M.  F.  The  loss  in  percentage,  in  any  circuit,  is  equal  to 
the  actual  loss  expressed  in  volts,  divided  by  the  line  voltage,  multiplied 
by  100;  or, 

P  =  -Q-  X  100. 
From  this  equation,  we  have: 

100 

If,  for  example,  the  calculation  is  to  be  made  on  a  loss  of  5  per  cent, 
with  an  applied  voltage  of  250,  using  this  last  equation,  we  would  have: 

5  X  250 
V  =  — -QQ- —  =  12.5  volts. 

P  F 
Substituting  the  equation      V=  -y^r-  in  Formula  1 ,  we  have: 


ELECTRIC  WIRING  27 

n  ,,        C  X  D  X  21.6 
— 


100 

C  X  D  X  21.6  X  100 
PE 

C  X  D  X  2,160 
P  E 

This  equation  it  should  be  remembered,  is  expressed  in  terms  of 
applied  voltage.  Now,  since  the  power  in  watts  is  equal  to  the  applied 
voltage  multiplied  by  the  current  (W  =  EC),  it  follows  that 

C--£- 
E 

By  substituting  this  value  of  C  in  the  equation  given  above  (  C  M— 

C  X  D  X  2  160\ 

*     -  |  ,  the  formula  is  expressed  in  terms  of  watts  instead 
r  L          / 

of  current,  thus: 

CM=  ^£V/-.  ............  (5) 

in  which  W  =  Power  in  watts  transmitted; 

D  =  Length  of  the  circuit  (one  way)  —  that  is,  the  length  of  one 

conductor; 

P  =  Figure  representing  the  percentage  loss; 
E*=  Applied  voltage. 

All  the  above  formulae  are  for  calculations  of  two-wire  circuits. 
In  making  calculations  for  three-wire  circuits,  it  is  usual  to  make  the 
calculation  on  the  basis  of  the  two  outside  conductors;  and  in  three 
wire  calculations,  the  above  formulae  can  be  used  with  a  slight  modifi- 
cation, as  will  be  shown. 

In  a  three-wire  circuit,  it  is  usually  assumed  in  making  the  cal- 
culation, that  the  load  is  equally  balanced  on  the  two  sides  of  the 
neutral  conductor;  and,  as  the  potential  across  the  outside  conductors 
is  double  that  of  the  corresponding  potential  across  a  two-  wire  circuit, 
it  is  evident  that  for  the  same  size  of  conductor  the  total  loss  in  volts 
could  be  doubled  without  increasing  the  percentage  of  loss  in  lamps. 
Furthermore,  as  the  load  on  one  side  of  the  neutral  conductor,  when 
the  system  is  balanced,  is  virtually  in  series  with  the  load  on  the 
third  side,  the  current  in  amperes  is  usually  one-half  the  sum  of  the 
current  required  by  all  the  lamps.  If  C  be  still  taken  as  the  total 

*NOTE.  Remember  that  V  in  Formulae  1  to  4  represents  the  volts  lost,  but  that 
E  in  Formula  5  represents  the  applied  voltage. 


28  ELECTRIC  WIRING 

current  in  amperes  (that  is,  the  sum  of  the  current  required  by  all  of 
the  lamps)  in  Formula  1,  we  shall  have  to  divide  this  current  by  2, 
to  use  the  formula  for  calculating  the  two  outside  conductors  for  a 
three-wire  system.  Furthermore,  we  shall  have  to  multiply  the 
voltage  lost  in  the  lamps  by  2,  to  obtain  the  voltage  lost  in  the  two  out- 
side conductors,  for  the  reason  that  the  potential  of  the  outside  con* 
ductors  is  double  the  potential  required  by  the  lamps  themselves. 
In  other  words,  Formula  1  will  become: 

CX  DX  21.6 


CM 


2  X  V  X  2 
CX  DX  21.6 


in  which  C  =  Sum  of  current  required  by  all  of  the  lamps  on  both  sides  of 

the  neutral  conductor; 

D  =  Length  of  circuit — that  is,  of  any  one  of  the  three  conductors; 
V  =  Loss  allowed  in  the  lamps,  i,  e.,  one-half  the  total  loss  in  the 
two  outside  conductors. 

In  the  same  manner,  all  of  the  other  formulse  may  be  adapted  for 
making  calculations  for  three-wire  systems.  Of  course  the  calcula- 
tion of  a  three-wire  system  could  be  made  as  if  it  were  a  two-wire 
system,  by  taking  one-half  the  total  number  of  lamps  supplied,  at 
one-half  the  voltage  between  the  outside  conductors. 

It  is  understood,  of  course,  that  the  size  of  the  conductor  in 
Formula  6  is  the  size  of  each  of  the  two  outs'de  ones;  but,  inasmuch 
as  the  Rules  of  the  National  Electric  Code  require  that  for  interior 
wiring  the  neutral  conductor  shall  be  at  least  equal  in  size  to  the  outside 
conductors,  it  is  not  necessary  to  calculate  the  size  of  the  neutral 
conductor.  It  must  be  remembered,  however,  that,  in  a  three-wire 
system  where  the  neutral  conductor  is  made  equal  in  capacity  to  the 
combined  size  of  the  two  outside  conductors,  and  where  the  two 
outside  conductors  are  joined  together,  we  have  virtually  a  two-wire 
system  arranged  so  that  it  can  be  converted  into  a  three- wire  system 
later.  In  this  case  the  calculation  is  exactly  the  same  as  in  the  case 
of  the  two-wire  circuits,  except  that  one  of  the  two  conductors  is  split 
into  two  smaller  wires  of  the  same  capacity.  This  is  frequently  done 
where  isolated  plants  are  installed,  and  where  the  generators  are  wound 
for  125  volts  and  it  may  be  desired  at  times  to  take  current  from  an 
outside  three-wire  125-250-volt  system. 


ELECTRIC  WIRING  29 

METHOD  OF  PLANNING  A  WIRING 
INSTALLATION 

The  first  step  in  planning  a  wiring  installation,  is  to  gather  all 
the  data  which  will  affect  either  directly  or  indirectly  the  system  of 
wiring  and  the  manner  in  which  the  conductors  are  to  be  installed. 
These  data  will  include:  Kind  of  building;  construction  of  building; 
space  available  for  conductors;  source  and  system  of  electric-current 
supply;  and  all  details  which  will  determine  the  method  of  wiring 
lo  be  employed.  These  last  items  materially  affect  the  cost  of  the 
work,  and  are  usually  determined  by  the  character  of  the  building 
and  by  commercial  considerations. 

Method  of  Wiring.  In  a  modern  fireproof  building,  the  only 
system  of  wiring  to  be  recommended  is  that  in  which  the  conductors 
are  installed  in  rigid  conduits;  although,  even  in  such  cases,  it  may  be 
desirable,  and  economy  may  be  effected  thereby,  to  install  the  larger 
feeder  and  main  conductors  exposed  on  insulators  using  weatherproof 
slow-burning  wire.  This  latter  method  should  be  used,  however, 
only  where  there  is  a  convenient  runway  for  the  conductors,  so  that 
they  will  not  be  crowded  and  will  not  cross  pipes,  ducts,  etc.,  and 
also  will  not  have  too  many  bends.  Also,  the  local  inspection  authori- 
ties should  be  consulted  before  usin^  this  method. 

For  mills,  factories,  etc.,  wires  exposed  on  cleats  or  insulators 
are  usually  to  be  recommended,  although  rigid  conduit,  flexible  con- 
duit, or  armored  cable  may  be  desirable. 

In  finished  buildings,  and  for  extensions  of  existing  outlets, 
where  the  wiring  could  not  readily  or  conveniently  be  concealed, 
moulding  is  generally  used,  particularly  where  cleat  wiring  or  other 
exposed  methods  of  wiring  would  be  objectionable.  However,  as 
has  already  been  said,  moulding  should  not  be  employed  where  there 
is  any  liability  to  dampness. 

In  finished  buildings,  particularly  where  they  are  of  frame  con- 
struction, flexible  steel  conduits  or  armored  cable  are  to  be  recom- 
mended. 

While  in  new  buildings  of  frame  construction,  knob  and  tube 
wiring  are  frequently  employed,  this  method  should  be  used  only 
where  the  question  of  first  cost  is  of  prime  importance.  While  armored 
cable  will  cost  approximately  50  to  100  per  cent  more  than  knob  and 


30  ELECTRIC  WIRING 

tube  wiring,  the  former  method  is  so  much  more  permanent  and  is 
so  much  safer  that  it  is  strongly  recommended. 

Systems  of  Wiring.  The  system  of  wiring — that  is,  whether 
the  two-wire  or  the  three-wire  system  shall  be  used — is  usually  deter- 
mined by  the  source  of  supply.  If  the  source  of  supply  is  an  isolated 
plant,  with  simple  two-wire  generators,  and  with  little  possibility 
of  current  being  taken  from  the  outside  at  some  future  time,  the 
wiring  in  the  building  should  be  laid  out  on  the  two-wire  system.  If, 
on  the  other  hand,  the  isolated  plant  is  three-wire  (having  three-wire 
generators,  or  two-wire  generators  with  balancer  sets),  or  if  the  cur- 
rent is  taken  from  an  outside  source,  the  wiring  in  the  building  should 
be  laid  out  on  a  three-wire  system. 

It  very  seldom  happens  that  current  supply  from  a  central  station 
is  arranged  with  other  than  the  three-wire  system  inside  of  buildings, 
because,  if  the  outside  supply  is  alternating  current,  the  transformers 
are  usually  adapted  for  a  three-wire  system.  For  small  buildings, 
on  the  other  hand,  where  there  are  only  a  few  lights  and  where  there 
would  be  only  one  feeder,  the  two-wire  system  is  used.  As  a  rule, 
however,  when  the  current  is  taken  from  an  outside  source,  it  is  best 
to  consult  the  engineer  of  the  central  station  supplying  the  current, 
and  to  conform  with  his  wishes.  As  a  matter  of  fact,  this  should  be 
done  in  any  event,  in  order  to  ascertain  the  proper  voltage  for  the 
lamps  and  for  the  motors,  and  also  to  ascertain  whether  the  central 
station  will  supply  transformers,  meters,  and  lamps — for,  if  these 
are  not  thus  supplied,  they  should  be  included  in  the  contract  for  the 
wiring. 

Location  of  Outlets.  It  is  not  within  the  scope  of  this  treatise 
to  discuss  the  matter  of  illumination,  but  it  is  desirable,  at  this  point, 
to  outline  briefly  the  method  of  procedure. 

A  set  of  plans,  including  elevation  and  details,  if  any,  and  show- 
ing decorative  treatment  of  the  various  rooms,  should  be  obtained 
from  the  Architect.  A  careful  study  should  then  be  made  by  the 
Architect,  the  Owner,  and  the  Engineer,  or  some  other  person  qualified 
to  make  recommendations  as  to  illumination.  The  location  of  the 
outlets  will  depend:  First,  upon  the  decorative  treatment  of  the 
room,  which  determines  the  aesthetic  and  architectural  effects;  second* 
upon  the  type  and  general  form  of  fixtures  to  be  used,  which  should 
be  previously  decided  on;  third,  upon  the  tastes  of  the  owners  or 


ELECTRIC  WIRING  31 

occupants  in  regard  to  illumination  in  general,  as  it  is  found  that 
tastes  vary  widely  in  regard  to  amount  and  kind  of  illumination. 

The  location  of  the  outlets,'  and  the  number  of  lights  required 
at  each,  having  been  determined,  the  outlets  should  be  marked  on 
the  plans. 

The  Architect  should  then  be  consulted  as  to  the  location  of  the 
centers  of  distribution,  the  available  points  for  the  risers  or  feeders, 
and  the  available  space  for  the  branch  circuit  conductors. 

In  regard  to  the  rising  points  for  the  feeders  and  mains,  the  fol- 
lowing precautions  should  be  used  in  selecting  chases : 

1.  The  space  should  be  amply  large  to  accommodate  all  the  feeders  and 
mains  likely  to  rise  at  that  given  point.     This  seems  trite  and  unnecessary, 
but  it  is  the  most  usual  trouble  with  chases  for  risers.      Formerly  architects 
and  builders  paid  little  attention  to  the  requirements  for  chases  for  electrical 
work;  but  in  these  later  days  of  2-inch  and  2^-inch  conduit,  they  realize  that 
these  pipes  are  not  so  invisible  and  mysterious  as  the  force  they  serve  to  dis- 
tribute, particularly  when  twenty  or  more  such  conduits  must  be  stowed  away 
in  a  building  where  no  special  provision  has  been  made  for  them. 

2.  If  possible,   the  space  should  be  devoted  solely  to  electric  wiring. 
Steam  pipes  are  objectionable  on  account  of  their  temperature;  and  these  and 
all  other  pipes  are  objectionable  in  the  same  space  occupied  by  the  electrical 
conduits,  for  if  the  space  proves  too  small,  the  electric  conduits  are  the  first  to 
be  crowded  out. 

The  chase,  if  possible,  should  be  continuous  from  the  cellar  to  the  roof, 
or  as  far  as  needed.  This  is  necessary  in  order  to  avoid  unnecessary  bends  or 
elbows,  which  are  objectionable  for  many  reasons. 

In  similar  manner,  the  location  of  cut-out  cabinets  or  distributing 
centers  should  fulfil  the  following  requirements: 

1.  They  should  be  accessible  at  all  times. 

2.  They  should  be  placed  sufficiently  close  together  to  prevent  the  cir- 
cuits from  being  too  long. 

3.  Do  not  place  them  in  too  prominent  a  position,  as  that  is  objectionable 
from  the  Architect's  point  of  view. 

4.  They  should  be    placed  as  near  as  possible  to  the   rising  chases,  in 
order  to  shorten  the  feeders  and  mains  supplying  them. 

Having  determined  the  system  and  method  of  wiring,  the  location 
of  outlets  and  distributing  centers,  the  next  step  is  to  lay  out  the  branch 
circuits  supplying  the  various  outlets. 

Before  starting  to  lay  out  the  branch  circuits,  a  drawing  showing 
the  floor  construction,  and  showing  the  space  between  the  top  of  the 
beams  and  girders  and  the  flooring,  should  be  obtained  from  the  Archi- 
tect. In  f  "-proof  buildings  of  iron  or  steel  construction,  it  is  almost 
the  invariable  practice,  where  the  work  is  to  be  concealed,  to  run  *he 


32 


ELECTRIC  WIRING 


conduits  ove*  the  beams,  under  the  rough  flooring,  carrying  them 
between  the  sleepers  when  running  parallel  to  the  sleepers,  and  notch- 
ing the  latter  when  the  conduits  run  across  them  (see  Fig.  31).  In 
wooden  frame  buildings,  the  conduits  run  parallel  to  the  beams  and 
to  the  furring  (see  Fig.  32);  they  are  also  sometimes  run  below  the 

Finished  Floorv 


Fig.  31.    Running  Conductors  Concealed  under  Floor  in  Fireproof  Building. 

beams.  In  the  latter  case  the  beams  have  to  be  notched,  and  this  is 
allowable  only  in  certain  places,  usually  near  the  points  where  the 
beams  are  supported.  The  Architect's  drawing  is  therefore  necessary 
in  order  that  the  location  and  course  of  the  conduits  may  be  indicated 
on  the  plans. 

The  first  consideration  in  laying  out  the  branch  circuit  is  the 
number  of  outlets  and  number  of  lights  to  be  wired  on  any  one  branch 
circuit.  The  Rules  of  the  National  Electric  Code  (Rule  21-D)  require 
that  "no  set  of  incandescent  lamps  requiring  more  than  660  watts, 
whether  grouped  on  one  fixture  or  on  several  fixtures  or  pendants, 
will  be  dependent  on  one  cut-out."  While  it  would  be  possible  to 
have  branch  circuits  supplying  more  than  660  watts,  by  placing  various 
cut-outs  at  different  points  along  the  route  of  the  branch  circuit,  so 
as  to  subdivide  it  into  small  sections  to  comply  with  the  rule,  this 
method  is  not  recommended,  except  in  certain  cases,  for  exposed  wiring 
in  factories  or  mills.  As  a  rule,  the  proper  method  is  to  have  the 
cut-outs  located  at  the  center  of  distribution,  and  to  limit  each  branch 
circuit  to  660  watts,  which  corresponds  to  twelve  or  thirteen  50-watt 
lamps,  twelve  being  the  usual  limit.  Attention  is  called  to  the  fact 
that  the  inspectors  usually  allow  50  watts  for  each  socket  connected 
to  a  branch  circuit;  and  although  8-candle-power  lamps  may  be 
placed  at  some  of  the  outlets,  the  inspectors  hold  that  the  standard 
lamp  is  approximately  50  watts,  and  for  that  reason  th^T?  is  always 
the  likelihood  of  a  lamp  of  that  capacity  being  used,  and  their  mspec- 


ELECTRIC  WIRING 


33 


tion  is  based  on  that  assumption.  Therefore,  to  comply  with  the 
requirements,  an  allowance  of  not  more  than  twelve  lamps  per  branch 
circuit  should  be  made. 

In  ordinary  practice,  however,  it  is  best  to  reduce  this  number 
still  further,  so  as  to  make  allowance  for  future  extensions  or  to  increase 
the  number  of  lamps  that  may  be  placed  at  any  outlet.  For  this 
reason,  it  is  wise  to  keep  the  number  of  the  outlets  on  a  circuit  at  the 
lowest  point  consistent  with  economical  wiring.  It  has  been  proven 
by  actual  practice,  that  the  best  results  are  obtained  by  limiting  the 
number  to  five  or  six  outlets  on  a  branch  circuit.  Of  course,  where 
all  the  outlets  have  a  single  light  each,  it  is  frequently  necessary,  for 
reasons  of  economy,  to  increase  this  number  to  eight,  ten,  and,  in 
some  cases,  twelve  outlets. 

We  have  already  referred  to  the  location  of  the  wires  or  conduits. 
This  question  is  generally  settled  by  the  peculiarities  of  the  construc- 
tion of  the  building.  It  is  necessary  to  know  this,  however,  before 
laying  out  the  circuit  work,  as  it  frequently  determines  the  course  of 
a  circuit. 

Now,  as  to  the  course  of  the  circuit  work,  little  need  be  said, 
as  it  is  largely  influenced  by  the  relative  position  of  the  outlets,  cut- 


Stud  or 
Wall 


Wooden  Beam 
Furring  Strips 


Rough  Flooring" 
/Con  du.1  t 


Stud  or 
Wall 


Fig.  32.    Running  Conductors  Concealed  under  Floor  in  Wooden  Frame  Building. 

outs,  switches,  etc.  Between  the  cut-out  box  and  the  first  outlet,  and 
between  the  outlets,  it  will  have  to  be  decided,  however,  whether 
the  circuits  shall  run  at  right  angles  to  the  walls  of  the  building  or 
room,  or  whether  they  shall  run  direct  from  one  point  to  another, 
irrespective  of  the  angle  they  make  to  the  sleepers  or  beams.  Of 
course,  in  the  latter  case,  the  advantages  are  that  the  cost  is  some- 
what less  and  the  number  of  elbows  and  bends  is  reduced.  If  the 


34  ELECTRIC  WIRING 

tubes  are  bent,  however,  instead  of  using  elbows,  the  difference  in 
cost  is  usually  very  slight,  and  probably  does  not  compensate  for  the 
disadvantages  that  would  result  from  running  the  tubes  diagonally. 
As  to  the  number  of  bends,  if  branch  circuit  work  is  properly  laid 
out  and  installed,  and  a  proper  size  of  tube  used,  it  rarely  happens 
that  there  is  any  difference  in  "pulling"  the  branch  circuit  wires. 
It  may  happen,  in  the  event  of  a  very  long  run  or  one  having  a  large 
number  of  bends,  that  it  might  be  advisable  to  adopt  a  short  and 
most  direct  route. 

Up  to  this  time,  the  location  of  the  distribution  centers  has  been 
made  solely  with  reference  to  architectural  considerations;  but  they 
must  now  be  considered  in  conjunction  with  the  branch  circuit  work. 

It  frequently  happens  that,  after  running  the  branch  circuits 
on  the  plans,  we  find,  in  certain  cases,  that  the  position  of  centers  of 
distribution  may  be  changed  to  advantage,  or  sometimes  certain 
groups  may  be  dispensed  with  entirely  and  the  circuits  run  to  other 
points.  We  now  see  the  wisdom  of  ascertaining  from  the  Architect 
where  cut-out  groups  may  be  located,  rather  than  selecting  particular 
points  for  their  location. 

As  a  rule,  wherever  possible,  it  is  wise  to  limit  the  length  of  each 
branch  circuit  to  100  feet;  and  the  number  and  location  of  the  dis- 
tributing centers  should  be  determined  accordingly. 

It  may  be  found  that  it  is. sometimes  necessary  and  even  desirable 
to  increase  the  limit  of  length.  One  instance  of  this  may  be  found  in 
hall  or  corridor  lights  in  large  buildings.  It  is  generally  desirable, 
in  such  cases,  to  control  the  hall  lights  from  one  point;  and,  as  the 
number  of  lights  at  each  outlet  is  generally  small,  it  would  not  be 
econonrcaJ.  to  run  mains  for  sub-centers  of  distribution.  Hence, 
in  instances  of  this  character,  the  length  of  runs  will  frequently  exceed 
the  limit  named.  In  the  great  majority  of  cases,  however,  the  best 
results  are  obtained  by  limiting  the  runs  to  90  or  100  feet. 

.  There  are  several  good  reasons  for  placing  such  a  limit  on  the 
length  of  a  branch  circuit.  To  begin  with,  assuming  that  we  are  going 
to  place  a  limit  on  the  loss  in  voltage  (drop)  from  the  switchboard  to 
the  lamp,  it  may  be  easily  proven  that  up  to  a  certain  reasonable 
limit  it  is  more  economical  to  have  a  larger  number  of  distributing 
centers  and  shorter  branch  circuits,  than  to  have  fewer  centers  and 
longer  circuits.  It  is  usual,  in  the  better  class  of  work,  to  limit  the 


ELECTRIC  WIRING  35 

loss  in  voltage  in  any  branch  circuit  to  approximately  one  volt.  As- 
suming this  limit  (one  volt  loss),  it  can  readily  be  calculated  that  the 
number  of  lights  at  one  outlet  which  may  be  connected  on  a  branch 
circuit  100  feet  long  (using  No.  14  B.  &  S.  wire),  is  jour]  or  in  the 
case  of  outlets  having  a  single  light  each,  -five  outlets  may  be  con- 
nected on  the  circuit,  the  first  being  60  feet  frcrn  the  cut-out,  the  others 
being  10  feet  apart. 

These  examples  are  selected  simply  to  show  that  if  the  branch 
circuits  are  much  longer  than  100  feet,  the  loss  must  be  increased 
to  more  than  one  volt,  or  else  the  number  of  lights  that  may  be  con- 
nected to  one  circuit  must  be  reduced  to  a  very  small  quantity,  pro- 
vided, of  course,  the  size  of  the  wire  remains  the  same. 

Either  of  these  alternatives  is  objectionable — the  first,  on  the 
score  of  regulation;  and  the  second,  from  an  economical  standpoint. 
If,  for  instance,  the  loss  in  a  branch  circuit  with  all  the  lights  turned 
on  is  four  volts  (assuming  an  extreme  case),  the  voltage  at  which  a 
lamp  on  that  circuit  burns  will  vary  from  four  volts,  depending  on  the 
number  of  lights  burning  at  a  time.  This,  of  course,  will  cause  the 
lamp  to  burn  below  candle-power  when  all  the  lamps  are  turned  on, 
or  else  to  diminish  its  life  by  burning  above  the  proper  voltage  when 
it  is  the  only  lamp  burning  on  the  circuit.  Then,  too,  if  the  drop  in 
the  branch  circuits  is  increased,  the  sizes  of  the  feeders  and  the  mains 
must  be  correspondingly  increased  (if  the  total  loss  remains  the  same), 
thereby  increasing  their  cost. 

If  the  number  of  lights  on  the  circuit  is  decreased,  we  do  not  use 
to  good  advantage  the  available  carrying  capacity  of  the  wire. 

Of  course,  one  solution  of  the  problem  would  be  to  increase  the 
size  of  the  wire  for  the  branch  circuits,  thus  reducing  the  drop.  This, 
however,  would  not  be  desirable,  except  in  certain  cases  where  there 
were  a  few  long  circuits,  such  as  for  corridor  lights  or  other  special 
control  circuits.  In  such  instances  as  these,  it  would  be  better  to 
increase  the  sizes  of  the  branch  circuit  to  No.  12  or  even  No.  10 
B.  &  S.  Gauge  conductors,  than  to  increase  the  number  of  centers 
3f  distribution  for  the  sake  of  a  few  circuits  only,  in  order  to  reduce 
the  number  of  lamps  (or  loss)  within  the  limit. 

The  method  of  calculating  the  loss  in  conductors  has  been  given 
elsewhere;  but  it  must  be  borne  in  mind,  in  calculating  the  loss  of  a 
branch  circuit  supplying  more  than  one  outlet,  that  separate  calcu- 


36  ELECTRIC  WIRING 

lations  must  be  made  for  each  portion  of  the  circuit.  That  is,  a 
calculation  must  be  made  for  the  loss  to  the  first  outlet,  the  length  in 
this  case  being  the  distance  from  the  center  of  distribution  to  the  first 
outlet,  and  the  load  being  the  total  number  of  lamps  supplied  by  the 
circuit.  The  next  step  would  be  to  obtain  the  loss  between  the  first 
and  second  outlet,  the  length  being  the  distance  between  the  two  out- 
lets, and  the  load,  in  this  case,  being  the  total  number  of  lamps  sup- 
plied by  the  circuit,  minus  the  number  supplied  by  the  first  outlet; 
and  so  on.  The  loss  for  the  total  circuit  would  be  the  sum  of  these 
losses  for  the  various  portions  of  the  circuit. 

Feeders  and  Mains.  If  the  building  is  more  than  one  story,  an 
elevation  should  be  made  showing  the  height  and  number  of  stories. 
On  this  elevation,  the  various  distributing  centers  should  be  shown 
diagrammatically ;  and  the  current  in  amperes  supplied  through 
each  center  of  distribution,  should  be  indicated  at  each  center.  The 
next  step  is  to  lay  out  a  tentative  system  of  feeders  and  mains,  and  to 
ascertain  the  load  in  amperes  supplied  by  each  feeder  and  main. 
The  estimated  length  of  each  feeder  and  main  should  then  be  deter- 
mined, and  calculation  made  for  the  loss  from  the  switchboard  to 
each  center  of  distribution.  It  may  be  found  that  in  some  cases  it 
will  be  necessary  to  change  the  arrangement  of  feeders  or  mains,  or 
even  the  centers  of  distribution,  in  order  to  keep  the  total  loss  from  the 
switchboard  to  the  lamps  within  the  limits  previously  determined. 
As  a  matter  of  fact,  in  important  work,  it  is  always  best  to  lay  out  the 
entire  work  tentatively  in  a  more  or  less  crude  fashion,  according  to 
the  "cut  and  dried"  method,  in  order  to  obtain  the  best  results,  because 
the  entire  layout  may  be  modified  after  the  first  preliminary  layout 
has  been  made.  Of  course,  as  one  becomes  more  experienced  and 
skilled  in  these  matters,  the. final  layout  is  often  almost  identical  with 
the  first  preliminary  arrangement. 

TESTING 

Where  possible,  two  tests  of  the  electric  wiring  equipment  should 
be  made,  one  after  the  wiring  itself  is  entirely  completed,  and  switches, 
cut-out  panels,  etc.,  are  connected;  and  the  second  one  after  the 
fixtures  have  all  been  installed.  The  reason  for  this  is  that  if  a  ground 
or  short  circuit  is  discovered  before  the  fixtures  are  installed,  it  is 
more  easily  remedied;  and  secondly,  because  there  is  no  division  oi 


ELECTRIC  WIRING  37 

the  responsibility,  as  there  might  be  if  the  first  test  were  made  only 
after  the  fixtures  were  installed.  If  the  test  shows  no  grounds  or 
short  circuits  before  the  fixtures  are  installed,  and  one  does  develop 
after  they  are  installed,  the  trouble,  of  course,  is  that  the  short  circuit 
or  ground  is  one  or  more  of  the  fixtures.  As  a  matter  of  fact,  it  is  a 
wise  plan  always  to  make  a  separate  test  of  each  fixture  after  it  is 
delivered  at  the  building  and  before  it  is  installed. 

While  a  magneto  is  largely  used  for  the  purpose  of  testing,  it  is 
at  best  a  crude  and  unreliable  method.  In  the  first  place,  it  does 
not  give  an  indication,  even  approximately,  of  the  total  insulation 
resistance,  but  merely  indicates  whether  there  is  a  ground  or  short 
circuit,  or  not.  In  some  instances,  moreover,  a  magneto  test  has 
led  to  serious  errors,  for  reasons  that  will  be  explained.  If,  as  is 
nearly  always  the  case,  the  magneto  is  an  alternating-current  instru- 
ment, it  may  sometimes  happen — particularly  in  long  cables,  and 
especially  where  there  is  a  lead  sheathing  on  the  cable — that  the 
magneto  will  ring,  indicating  to  the  uninitiated  that  there  is  a  ground 
or  short  circuit  on  the  cable.  This  may  be,  and  usually  is,  far  from 
being  the  case;  and  the  cause  of  the  ringing  of  the  magneto  is  not  a 
ground  or  short  circuit,  but  is  due  to  the  capacity  of  the  cable,  which 
acts  as  a  condenser  under  certain  conditions,  since  the  magneto  produc- 
ing an  alternating  current  repeatedly  charges  and  discharges  the  cable 
in  opposite  directions,  this  changing  of  the  current  causing  the  magneto 
to  ring.  Of  course,  this  defect  in  a  magneto  could  be  remedied  by 
using  a  commutator  and  changing  it  to  a  direct-current  machine; 
but  as  the  method  is  faulty  in  itself,  it  is  hardly  worth  while  to  do  this. 

A  portable  galvanometer  with  a  resistance  box  and  Wheatstone 
bridge,  is  sometimes  employed;  but  this  method  is  objectionable 
because  it  requires  a  special  instrument  which  cannot  be  used  for 
many  other  purposes.  Furthermore,  it  requires  more  skill  and  time 
to  use  than  the  voltmeter  method,  which  will  now  be  described. 

The  advantage  of  the  voltmeter  method  is  that  it  requires  merely 
a  direct-current  voltmeter,  which  can  be  used  for  many  other  purposes, 
and  which  all  engineers  or  contractors  should  possess,  together  with 
a  box  of  cells  having  a  potential  of  preferably  over  30  volts.  The  volt- 
meter should  have  a  scale  of  not  over  150  volts,  for  the  reason  that  if 
the  scale  on  which  the  battery  is  used  covers  too  wide  a  range  (say 
1,000  volts)  the  readings  might  be  so  small  as  to  make  the  test  inac- 


38 


ELECTRIC  WIRING 


curate.  A  good  arrangement  would  be  to  have  a  voltmeter  Having 
two  scales — say,  one  of  60  and  one  of  600 — which  would  make  the 
voltmeter  available  for  all  practical  potentials  that  are  likely  to  be 
used  inside  of  a  building.  If  desired,  a  voltmeter  could  be  obtained 
with  three  connections  having  three  scales,  the  lowest  scale  of  which 
would  be  used  for  testing  insulation  resistances. 

Before  starting  a  test,  all  of  the  fuses  should  be  inserted  and 
switches  turned  on,  so  that  the  complete  test  of  the  entire  installation 
can  be  made.  When  this  has  been  done,  the  voltmeter  and  battery 
should  be  connected,  so  as  to  obtain  on  the  lowest  scale  of  the  volt- 
meter the  electromotive  force  of  the  entire  group  of  cells.  This 
connection  is  shown  in  Fig.  33.  Immediately  after  this  has  been  done, 

the  insulation  resistance  to  be  tested 
is  placed  in  circuit,  whether  the 
insulation  to  be  tested  is  a  switch- 
board, slate  panel-board,  or  the 
entire  wiring  installation;  and  the 
connections  are  made  as  shown  in 
Fig.  34.  A  reading  should  then 
again  be  taken  of  the  voltmeter; 
and  the  leakage  is  in  proportion 
to  the  difference  between  the  first 
and  second  readings  of  the  volt- 
meter. The  explanation  given  below 

will  show  how  this  resistance  may  be  calculated:  It  is  evident  that 
the  resistance  in  the  first  case  was  merely  the  resistance  of  the  volt- 
meter and  the  internal  resistance  of  the  battery.  As  a  rub,  the  internal 
resistance  of  the  battery  is  so  small  in  comparison  with  the  resistance 
of  the  voltmeter  and  the  external  resistance,  that  it  may  be  entirely 
neglected,  and  this  will  be  done  in  the  following  calculation.  In  the 
second  case,  however,  the  total  resistance  in  circuits  is  the  resistance 
of  the  voltmeter  and  the  battery,  plus  the  entire  insulation  resistance 
on  all  the  wires,  etc.,  connected  in  circuit. 

To  put  this  in  mathematical  form,  the  voltage  of  the  cells  may 
be  indicated  by  the  letter  E;  and  the  reading  of  the  voltmeter  when 
the  insulation  resistance  is  connected  by  the  circuit,  by  the  letter  E'. 
Let  R  represent  the  resistance  of  the  voltmeter  and  Rx  represent  the 
insulation  resistance  of  the  installation  which  we  wrish  to  measure. 


Fig.  33.     Connections  of  Voltmeter  and 

Battery  for  Testing  Insulation 

Resistance. 


ELECTRIC  WIRING 


39 


It  is  a  fact  which  the  reader  undoubtedly  knows,  that  the  E.  M.  F.  as 
indicated  by  the  voltmeter  in  Fig.  34  is  inversely  proportional  to  the 
resistance:  that  is,  the  greater  the  resistance,  the  lower  will  be  the 
reading  on  the  voltmeter,  as  this  reading  indicates  the  leakage  or  cur- 
rent passing  through  the  resistance.  Putting  this  in  the  shape  of  a 
formula,  we  have  from  the  theory  of  proportion : 

E  :  E'  ::  R  +  Rx  :  R  ; 
or, 

Transposing, 
and 


;'  R  +  E'  Rx  =  E  R . 
7  RX  =  E  R  -  E'  R  =  R  (E-E'\ 
R(E-  E'} 


Or,  expressed  in  words,  the  insulation  resistance  is  equal  to  the  resist- 
ance of  the  volt- 
meter multiplied  by 
the  difference  be- 
tween the  first  read- 
ing (or  the  voltage 
in  the  cells)  and 
the  second  reading 
(or  the  reading  of 
the  voltmeter  with 
the  insulation  re- 
sistance in  series  with  the  voltmeter),  divided  by  this  last  reading  of 
the  voltmeter. 

Example.  Assume  a  resistance  of  a  voltmeter  (R)  of  20,000  ohms, 
and  a  voltage  of  the  cells  (E)  of  30  volts;  and  suppose  that  the  insula- 
tion resistance  test  of  a  wiring  installation,  including  switchboard, 
feeders,  branch  circuits,  panel-boards,  etc.,  is  to  be  made,  the  insula- 
tion resistance  being  represented  by  the  letter  Rx .  By  substituting 

in  the  formula 

R(E-  E'} 


+BUS 


Fig.  34.    Insulation  Resistance  Placed  in  Circuit,  Ready  :or 
Testing. 


Rx  = 


E' 


and  assuming  that  the  reading  of  the  voltmeter  with  the  insulation 
resistance  connected  is  5,  we  have : 


R    = 


20,000  X  (30-5) 


100,000  ohms. 


If  the  test  shows  an  excessive  amount  of  leakage,  or  a  ground  or 


40  ELECTRIC  WIRING 

short  circuit,  the  location  of  the  trouble  may  be  determined  by  the 
process  of  elimination — that  is,  by  cutting  out  the  various  feeders 
until  the  ground  or  leakage  disappears,  and,  when  the  feeder  on  which 
the  trouble  exists  has  been  located,  by  following  the  same  process 
with  the  branch  circuits. 

Of  course,  the  larger  the  installation  and  the  longer  and  more 
numerous  the  circuits,  the  greater  the  leakage  will  be;  and  the  lower 
will  be  the  insulation  resistance,  as  there  is  a  greater  surface  exposed 
for  leakage.  The  Rules  oj  the  National  Electric  Code  give  a  sliding 
scale  for  the  requirements  as  to  insulation  resistance,  depending  upon 
the  amount  of  current  carried  by  the  various  feeders,  branch  circuits, 
etc.  The  rule  of  the  National  Electric  Code  (No.  66)  covering  this 
point,  is  as  follows: 

"The  wiring  in  any  building  must  test  free  from  grounds;  i.  e.,  the  com- 
plete installation  must  have  an  insulation  between  conductors  and  between 
all  conductors  and  the  ground  (not  including  attachments,  sockets,  recepta- 
cles, etc.)  not  less  than  that  given  in  the  following  table: 

Up  to          5  amperes 4,000,000  ohms 


10 

25 

50 

100 

200 

400 

800 

1,600 


2,000,000. 

800,000 

400,000 

200,000 

100,000 

50,000 

25,000 

12,500 


"The  test  must  be  made  with  all  cut-outs  and  safety  devices  in  place.  If 
the  lamp  sockets,  receptacles,  electroliers,  etc.,  are  also  connected,  only  one- 
half  of  the  resistances  specified  in  the  table  will  be  required." 

ALTERNATING-CURRENT  CIRCUITS 

It  is  not  within  the  province  of  this  chapter  to  treat  the  various 
alternating-current  phenomena,  but  simply  to  outline  the  modifications 
which  should  be  made  in  designing  and  calculating  electric  light 
wiring,  in  order  to  make  proper  allowance  for  these  phenomena. 

The  most  marked  difference  between  alternating  and  direct  cur- 
rent, so  far  as  wiring  is  concerned,  is  the  effect  produced  by  self- 
induction,  which  is  characteristic  of  all  alternating-current  circuits. 
This  self-induction  varies  greatly  with  conditions  depending  upon 
the  arrangement  of  the  circuit,  the  medium  surrounding  the  circuit, 
the  devices  or  apparatus  supplied  by  or  connected  in  the  circuit,  etc. 


ELECTRIC  WIRING  41 

For  example,  if  a  coil  having  a  resistance  of  100  ohms  is  included  in 
the  circuit,  a  current  of  one  ampere  can  be  passed  through  the  coil 
with  an  electric  pressure  of  100  volts,  if  direct  current  is  used ;  while 
it  might  require  a  potential  of  several  hundred  volts  to  pass  a  current 
of  one  ampere  if  alternating-current  were  used,  depending  upon  the 
number  of  turns  in  the  coil,  whether  it  is  wound  on  iron  or  some  other 
non-magnetic  material,  etc. 

It  will  be  seen  from  this  example,  that  greater  allowance  should 
be  made  for  self-induction  in  laying  out  and  calculating  alternating- 
current  wiring,  if  the  conditions  are  such  that  the  self-induction  will 
be  appreciable. 

On  account  of  self-induction,  the  two  wires  of  an  alternating- 
current  circuit  must  never  be  installed  in  separate  iron  or  steel  con- 
duits, for  the  reason  that  such  a  circuit  would  be  virtually  a  choke  coil 
consisting  of  a  single  turn  of  wire  wound  on  an  iron  core,  and  the  self- 
induction  would  not  only  reduce  the  current  passing  through  the  cir- 
cuit, but  also  might  produce  heating  of  the  iron  pipe.  It  is  for  this 
reason  that  the  National  Electric  Code  requires  conductors  constitut- 
ing a  given  circuit  to  be  placed  in  the  same  conduit,  if  that  conduit 
is  iron  or  steel,  whenever  the  said  circuit  is  intended  to  carry,  or  is 
liable  to  carry  at  some  future  time,  an  alternating  current.  This  does 
not  mean,  in  the  case  of  a  two-phase  circuit,  that  all  four  conductors 
need  be  placed  in  the  same  conduit,  but  that  the  two  conductors  of  a 
given  phase  must  be  placed  in  the  same  conduit.  If,  however,  the 
three-wire  system  be  used  for  a  two-phase  system,  all  three  conductors 
should  be  placed  in  the  same  conduit,  as  should  also  be  the  case  in  a 
three-wire  three-phase  system.  Of  course,  in  a  single-phase  two-  or 
three-wire  system,  the  conductors  should  all  be  placed  in  the  same 
conduit. 

In  calculating  circuits  carrying  alternating  current,  no  allowance 
usually  should  be  made  for  self-induction  when  the  conductors  of  the 
same  circuit  are  placed  close  together  in  an  iron  conduit.  When, 
however,  the  conductors  are  run  exposed,  or  are  separated  from  each 
other,  calculation  should  be  made  to  determine  if  the  effects  of  self- 
induction  are  great  enough  to  cause  an  appreciable  inductive  drop. 
There  are  several  methods  of  calculating  this  drop  due  to  self-induc- 
tion—one by  formula,  and  one  by  means  of  chart  and  table  which  will 
be  described. 


ELECTRIC  WIRING 


Skin  Effect.  Skin  effect  in  alternating-current  circuits  is  caused 
by  an  incorrect  distribution  of  the  current  in  the  wire,  the  current 
tending  to  flow  through  the  outer  portion  of  the  wire,  it  being  a  well- 
known  fact  that  in  alternating  currents,  the  current  density  decreases 
toward  the  center  of  the  conductor,  and  that  in  large  wires,  the  current 
density  at  the  center  of  the  conductor  is  relatively  quite  small. 

The  skin  effect  increases  in  proportion  to  the  square  of  the  diam- 
eter, and  also  in  direct  ratio  to  the  frequency  of  the  alternating  current. 

For  conductors  of  No.  0000  B.  &  S.  Gauge,  and  smaller,  and  for 
frequencies  of  60  cycles  per  second,  or  less,  .the  skin  effect  is  negligible 
and  is  less  than  one-half  of  one  per  cent. 

For  very  large  cables  and  for  frequencies  above  60  cycles  per 
second,  the  skin  effect  may  be  appreciable;  and  in  certain  cases,  allow- 
ance for  it  should  be  made  in  making  the  calculation.  In  ordinary 
practice,  however,  it  may  be  neglected.  Table  IX,  taken  from  Alter- 
nating-Current Wiring  and  Distribution,  by  W.  R.  Emmet,  gives  the 
data  necessary  for  calculating  the  skin  effect.  The  figures  given  in 
the  first  and  third  columns  are  obtained  by  multiplying  the  size  of  the 
conductor  (in  circular  mils)  by  the  frequency  (number  of  cycles  per 
second) ;  and  the  figures  in  the  second  and  fourth  columns  show  the 
factor  to  be  used  in  multiplying  the  ohmic  resistance,  in  order  to 
obtain  the  combined  resistance  and  skin  effect. 

TABLE  IX 
Data  for  Calculating  Skin  Effect 


PRODUCT   OF   CIRCULAR 
MILS  X  CYCLES  PER  SEC. 

FACTOR 

PRODUCT   op    CIRCULAR 
MILS  X  CYCLES  PER  SEC. 

FACTOR 

10,000,000 

.00 

70,000,000 

1.13 

20,000,000 

.01 

80,000,000 

1.17 

30,000,000 

.03 

90,000,000 

1.20 

40,000,000 

.05 

100,000,000 

1.25 

50,000,000 

.08 

125,000,000 

1.34 

60,000,000 

.10 

150,000,000 

1.43 

The  factors  given  in  this  table,  multiplied  by  the  resistance  to  direct  cur- 
rents, will  give  the  resistance  to  alternating  currents  for  copper  conductors  of 
circular  cross-section. 

Mutual  Induction.  When  two  or  more  circuits  are  run  in  the 
same  vicinity,  there  is  a  possibility  of  one  circuit  inducing  an  electro- 
motive force  in  the  conductors  of  an  adjoining  circuit.  This  effect 
may  result  in  raising  or  lowering  the  E.  M.  F.  in  the  circuit  in  which  a 


ELECTRIC  WIRING  43 

mutual  induction  takes  place.  The  amount  of  this  induced  E.  M.  F. 
set  up  in  one  circuit  by  a  parallel  current,  is  dependent  upon  the  cur- 
rent, the  frequency,  the  lengths  of  the  circuits  running  parallel  to  each 
other,  and  the  relative  positions  of  the  conductors  constituting  the 
said  circuits. 

Under  ordinary  conditions,  and  except  for  long  circuits  carrying 
high  potentials,  the  effect  of  mutual  induction  is  so  slight  as  to  be 
negligible,  unless  the  conductors  are  improperly  arranged.  In  order 
to  prevent  mutual  induction,  the  conductors  constituting  a  given 
circuit  should  be  grouped  together.  Figs.  35  to  39,  inclusive,  show 

0         0  te>ooo  Alt.       .035  Volts, 

O  O  7200     AH.         ,016  Volts, 

Fig.  35. 

O          O          •  •  I6>000  Alt-        -°'5  Volts. 

7,200     Alt.         .0065Volts. 
Fig.  36. 

A         r\         s-\         A  13000  Ait.       .070  Volts. 

7,800     Alt.         .038  Volts. 
Fig.  37. 

O/~\  A  fa  I^OOO  Alt.        .006 Volts. 

^  7200      Alt.          .0027  Volts. 

Fig.  38. 

0O  O          A  I6POO  Alt.        .112    Volts. 

^  7faoo   Alt       .0^0  Volts, 

Fig.  39. 

Various  Groupings  of  Conductors  in  Two  Two- Wire  Circuits,   Giving   Various 
Effects  of  Induction. 

five  arrangements  of  two  two-wire  circuits;  and  show  how  relatively 
small  the  effect  of  first  induction  is  when  the  conductors  are  properly 
arranged,  as  in  Fig.  38,  and  how  relatively  large  it  may  be  when  im- 
properly arranged,  as  in  Fig.  39.  These  diagrams  are  taken  from 
a  publication  of  Mr.  Charles  F.  Scott,  entitled  Polyphase  Trans- 
mission, issued  by  the  Westinghouse  Electric  &  Manufacturing 
Company. 

Line  Capacity.  The  effect  of  capacity  is  usually  negligible, 
except  in  long  transmission  lines  where  high  potentials  are  used ;  no 
calculations  or  allowance  need  be  made  for  capacity,  for  ordinary 
circuits. 


44  ELECTRIC  WIRING 

Calculation  of  Alternating=Current  Circuits.  In  the  instruction 
paper  on  'Tower  Stations  and  Transmission,"  a  method  is  given  for 
calculating  alternating-current  lines  by  means  of  formulae,  and  data  are 
given  regarding  power  factor  and  the  calculation  of  both  single-phase 
and  polyphase  circuits.  For  short  lines,  secondary  wiring,  etc.,  how- 
ever, it  is  probably  more  convenient  to  use  the  chart  method  devised 
by  Mr.  Ralph  D.  Mershon,  described  in  the  American  Electrician  of 
June,  1897,  and  partially  reproduced  as  follows: 

DROP  IN  ALTERNATINQ=CURRENT  LINES 

When  alternating  currents  first  came  into  use,  when  transmission 
distances  were  short  and  the  only  loads  carried  were  lamps,  the  ques- 
tion of  drop  or  loss  of  voltage  in  the  transmitting  line  was  a  simple  one, 
and  the  same  methods  as  for  direct  current  could  without  serious 
error  be  employed  in  dealing  with  it.  The  conditions  existing  in 
alternating  practice  to-day — longer  distances,  polyphase  circuits, 
and  loads  made  up  partly  or  wholly  of  induction  motors — render 
this  question  less  simple;  and  direct-current  methods  applied  to  it 
do  not  lead  to  satisfactory  results.  Any  treatment  of  this  or  of 
any  engineering  subject,  if  it  is  to  benefit  the  majority  of  engineers, 
must  not  involve  groping  through  long  equations  or  complex  diagrams 
in  search  of  practical  results.  The  results,  if  any,  must  be  in  avail- 
able and  convenient  form.  In  what  follows,  the  endeavor  has  b?en 
made  to  so  treat  the  subject  of  drop  in  alternating-current  lines  tliat 
if  the  reader  be  grounded  in  the  theory  the  brief  space  devoted  to 
it  will  suffice;  but  if  he  do  not  comprehend  or  care  to  follow  the 
simple  theory  involved,  he  may  nevertheless  turn  the  results  to  his 
practical  advantage. 

Calculation  of  Drop.  Most  of  the  matter  heretofore  published 
on  the  subject  of  drop  treats  only  of  the  inter-relation  of  the  E.  M.  F.  Js 
involved,  and,  so  far  as  the  writer  knows,  there  have  not  appeared 
in  convenient  form  the  data  necessary  for  accurately  calculating  this 
quantity.  Table  X  (page  47)  and  the  chart  (page  46)  include,  in  a 
form  suitable  for  the  engineer's  pocketbook,  everything  necessary 
for  calculating  the  drop  of  alternating-current  lines. 

The  cnart  is  simply  an  extension  of  the  vector  diagram  (Fig.  40), 
giving  the  relations  of  the  E.  M.  F.'s  of  line,  load  and  genera  tpi\  In 


ELECTRIC  WIRING 


45 


Fig.  40,  E  is  the  generator  E.  M.  F. ;  e,  the  E.  M.  F.  impressed  upon 
the  load ;  c,  that  component  of  E  which  overcomes  the  back  E.  M.  F. 
due  to  the  impedance  of  the  line.  The  component  c  is  made  up  of  two 
components  at  right  angles  to  each  other.  One  is  a,  the  component 
overcoming  the  IR  or  back  E.  M.  F.  due  to  resistance  of  the  line. 
The  other  is  6,  the  component  overcoming  the  reactance  E.  M.  F.  or 
back  E.  M.  F.  due  to  the  alternating  field  set  up  around  the  wire  by 
the  current  in  the  wire.  The  drop  is  the  difference  between  E  and 
e.  It  is  d,  the  radial  distance  between  two  circular  arcs,  one  of  which 
is  drawn  with  a  radius  e,  and  the  other  with  a  radius  E. 

The  chart  is  made  by  striking  a  succession  of  circular  arcs  with 
Oasacenter.  x 

The  radius  of  the 
smallest  circle  cor- 
responds to  e,  the 
E.  M.  F.  of  the 
load,  which  is  taken 
as  100  per  cent. 
The  radii  of  the  suc- 
ceeding circles  in- 
crease by  1  per  cent 
of  that  of  the  small- 
est circle;  and,  as 
the  radius  of  the 
last  or  largest  cir- 
cle  is  140  per  cent 
of  that  of  the  smallest,  the  chart  answers  for  drops  up  to  40  per  cent  of 
the  E.  M.  F.  delivered. 

The  terms  resistance  volts,  resistance  E.  M.  F.,  reactance  volts, 
and  reactance  E.  M.  F.,  refer,  of  course,  to  the  voltages  for  overcom- 
ing the  back  E.  M.  F.  Js  due  to  resistance  and  reactance  respectively. 
The  figures  given  in  the  table  under  the  heading  "Resistance-Volts 
for  One  Ampere,  etc."  are  simply  the  resistances  of  2,000  feet  of  the 
various  sizes  of  wire.  The  values  given  under  the  heading  "React- 
ance-Volts, ptc./'  are,  a  part  of  them,  calculated  from  tables  published 
some  time  ago  by  Messrs.  Houston  and  Kennelly.  The  remainder 
were  obtained  by  using  Maxwell's  formula. 

The  explanation  given  in  the  table  accompanying  the  chart 


Fig.  40.    Vector  Diagram. 


46 


ELECTRIC  WIRING 


9  .7 

LOAD  POWER 


O  10  20  3» 

DROP  IN  PERCENT  OF  E.1M.F.   DELIVERED 


Cnart  for  Calculating  Drop  in  Alternating-Current  Lines. 


ELECTRIC  WIRING 


47 


TABLE  X 
Data  for  Calculating  Drop  in  Alternating=Current  Lines 

To  be  used  in  conjunction  with  Chart  on  opposite  page. 

By  means  of  the  table,  calculate  the  Resistance- Volts  and  the  Reactance-Volts  in  the 
line,  and  find  what  per  cent  each  is  of  the  E.  M.  F.  delivered  at  the  end  of  the  line. 
Starting  from  the  point  on  the  chart  where  the  vertical  line  corresponding  with  the 
power-factor  of  the  load  intersects  the  smallest  circle,  lay  off  in  per  cent  the  resistance 
E.  M.  F.  horizontally  and  to  the  right;  from  the  point  thus  obtained,  lay  off  upward 
in  per  cent  the  reactance-E.  M.  F.  The  circle  on  which  the  last  point  falls  gives  the 
drop,  in  per  cent,  of  the  E.  M.  F.  delivered  at  the  end  of  the  line.  Every  tenth  circle 
arc  is  marked  with  the  per  cent  drop  *-o  which  it  corresponds. 


en 

m 

1 

0 

8 

en 

52 

"*"bC 
e«^ 

03  "^ 
O)  O 

!§, 

P  +^ 

||0 

PHW*O 

Throughout  the  table  the  lower  figures  in  the  squares  give 
values  for  ONE   MILE    of   line,  corresponding  to  those  of   the 
upper  figures  for  1,000  feet  of  line. 

Upper  figures  are  REACTANCE-VOLTS  in  1,000  ft.  of  Line  (= 
2,000  ft.  of   Wire)  for  One    Ampere  at  7,200   Alternations   per 
Minute  (60  Cycles  per  Second)   for  the  distance  given  between 
Centers  of  Conductors. 

w 

1" 

2" 

3" 

6" 

9" 

12" 

18" 

24" 

30" 

36" 

0000 

639 

3,376 

.098 

.518 

.046 

.243 

.079 

.417 

.111 

.586 

.130 
.687 

.161 

.850 

.180 

.951 

.193 

1.02 

.212 

1.12 

.225 

1.19 

.235 

1.24 

.244 

1.29 

000 
00 

507 

2,677 

.124 

.653 

.052 

.275 

.085 

.449 

.116 

.613 

.135 
.713 

.167 

.882 

.185 

.977 

.199 

1.05 

.217 

1.15 

.230 

1.22 

.241 

1.27 

.249 

1.32 

.254 

1.34 

.259 

1.37 

402 

2,123 

.156 

.824 

.057 

.301 

.090 

.475 

.121 

.639 

.140 
.739 

.172 

.908 

.190 

1.00 

.204 

1.08 

.222 

1.17 

.236 

1.25 

.246 

1.30 

0 

319 

1,685 

.197 

1.04 

.063 

.332 

.095 

.502 

.127 

.671 

.145 

.766 

.177 

.935 

.196 

1.04 

.209 

1.10 

.228 

1.20 

.233 

1.23 

.238 

1.26 

.241 

1.27 

.251 

1.33 

1 

2 

253 

1,335 

.248 

1.31 

.068 

.359 

.101 

.533 

.132 

.687 

.151 

.797 

.183 

.966 

.201 

1.06 

.214 

1.13 

.246 

1.30 

.256 

1.35 

.265 

1.40 

201 

1,059 

.313 

1.65 

.074 

.391 

.106 
.560 

.138 

.728 

.156 

.824 

.188 

.993 

.206 

1.09 

.220 

1.16 

.252 

1.33 

.262 

1.38 

.270 

1.43 

3 

159 

840 

.394 

2.08 

.079 

.417 

.112 

.591 

.143 

.755 

.162 
.856 

.193 

1.02 

.212 

1.12 

.225 

1.19 

.244 

1.29 

.257 

1.36 

.267 

1.41 

.275 

1.45 

.281 

1.48 

4 

126 

666 

.497 

2.63 

.085 

.449 

.117 

.618 

.149 

.787 

.167 

.882 

.199 

1.05 

.217 

1.15 

.230 

1.22 

.249 

1.32 

.254 

1.34 

.262 

1.38 

.272 

1.44 

5 

100 

528 

.627 

3.31 

.090 

.475 

.121 

.639 

.154 
.813 

.172 

.908 

.204 

1.08 

.223 

1.18 

.236 

1.25 

.268 

1.42 

.278 

1.47 

.286 

1.51 

6 

79 

419 

.791 

4.18 

.095 

.502 

.127 

.671 

.158 
.834 

.178 

.940 

.209 

1.10 

.214 

1.13 

.228 

1.20 

.233 

1.23 

.241 

1.27 

.246 

1.30 

.260 

1.37 

.265 

1.40 

.270 

1.43 

.272 

1.44 

.278 

1.47 

.283 

1.49 

,291 

1.54 

.296 

1.56 

.302 

1.60 

7 

63 

332 

.997 

5.27 

.101 

.533 

.132 

.697 

.164 

.866 

.183 

.966 

.288 

1.52 

8 

50 

263 

1.260 

6.64 

.106 
.560 

.138 

.729 

.169 

.893 

.188 

.993 

.220 

1.16 

.238 

1.26 

.252 

1  33 

.284 

1.50 

.293 

1.55 

48  ELECTRIC  WIRING 

(Table  X)  is  thought  to  be  a  sufficient  guide  to  its  use,  but  a  few 
examples  may  be  of  value. 

Problem.  Power  to  be  delivered,  250  K.W.  ;  E.  M.  F.  to  be  delivered, 
2,000  volts;  distance  of  transmission,  10,000  feet;  size  of  wire,  No.  0;  distance 
between  wires,  18  inches;  power  factor  of  load,  .8;  frequency,  7,200  alterna- 
tions per  minute.  Find  the  line  loss  and  drop. 

Remembering  that  the  power  factor  is  that  fraction  by  which 
the  apparent  power  of  volt-amperes  must  be  multiplied  to  give  the 
true  power,  the  apparent  power  to  be  delivered  is 

250  K.W.  A__w 

-  =312.5  apparent  K.W, 

.8 

The  current,  therefore,  at  2,000  volts  will  be 

312,500     1K.  0_ 
_-:=156.25  amperes. 

From  the  table  of  reactances  under  the  heading  "18  inches,"  and 
corresponding  to  No.  0  wire,  is  obtained  the  constant  .228.  Bearing 
the  instructions  of  the  table  in  mind,  the  reactance-volts  of  this  line 
are,  156.25  (amperes)  X  10  (thousands  of  feet)  X  .228=356.3  volts, 
which  is  17.8  per  cent  of  the  2,000  volts  to  be  delivered. 

From  the  column  headed  "Resistance-Volts"  and  corresponding 
to  No.  0  wire,  is  obtained  the  constant  .197.  The  resistance-volts 
of  the  line  are,  therefore,  156.25  (amperes)  X  10  (thousands  of  feet) 
X  .197=307.8  volts,  which  is  15.4  per  cent  of  the  2,000  volts  to  be 
delivered. 

Starting,  in  accordance  with  the  instructions  of  the  table,  from 
the  point  where  the  vertical  line  (which  at  the  bottom  of  the  chart 
is  marked  "Load  Power  Factor"  .8)  intersects  the  inner  or  smallest 
circle,  lay  off  horizontally  and  to  the  right  the  resistance-E.  M.  F.  in 
per  cent  (15  .4)  ;  and  from  the  point  thus  obtained,  lay  off  vertically  the 
reactance-E.  M.  F.  in  per  cent  (17.8).  The  last  point  falls  at  about 
23  per  cent,  as  given  by  the  circular  arcs.  This,  then,  is  the  drop,  in 
per  cent,  of  the  E.  M.  F.  delivered.  The  drop,  in  per  cent,  of  the  genera- 
tor E.  M.  F.  is,  of  course, 

23 

8.7  per  cent. 


The  percentage  loss  of  power  in  the  line  has  not,  as  with  direct 
current,  the  same  value  as  the  percentage  drop.  This  is  due  to  the 
fact  that  the  line  has  reactance,  and  also  that  the  apparent  power 


ELECTRIC  WIRING  49 

delivered  to  the  load  is  not  identical  with  the  true  power  —  that  is, 
the  load  power  factor  is  less  than  unity.  The  loss  must  be  obtained 
by  calculating  P  R  for  the  line,  or,  what  amounts  to  the  same  thing, 
by  multiplying  the  resistance-volts  by  the  current. 

The  resistance-volts  in  this  case  are  307.8,  and  the  current 
156.25  amperes.  The  loss  b  307.8  X  156.25  =  48.1  K.  W.  The 
percentage  loss  is 

48.1 


250+48.1 

Therefore,  for  the  problem  taken,  the  drop  is  18.7  per  cent,  and  the 
loss  is  16  .  1  per  cent.  If  the  problem  be  to  find  the  size  wire  for  a  given 
drop,  it  must  be  solved  by  trial.  Assume  a  size  of  wire  and  calculate 
the  drop  ;  the  result  in  connection  with  the  table  will  show  the  direction 
and  extent  of  the  change  necessary  in  the  size  of  wire  to  give  the 
required  drop. 

The  effect  of  the  line  reactance  in  increasing  the  drop  should  be 
noted.  If  there  were  no  reactance,  the  drop  in  the  above  example 
would  be  given  by  the  point  obtained  in  laying  off  on  the  chart  the 
resistance-E.  M.  F.  (15.4)  only.  This  point  falls  at  12.4  per  cent, 
and  the  drop  in  terms  of  the  generator  E.  M.  F.  would  be 

12  4 

•      '     =11  per  cent,  instead  of  18  .  7  per  cent. 
11.2  .4 

Anything  therefore  which  will  reduce  reactance  is  desirable. 

Reactance  can  be  reduced  in  two  ways.  One  of  these  is  to 
diminish  the  distance  between  wires.  The  extent  to  which  this  can 
be  carried  is  limited,  in  the  case  of  a  pole  line,  to  the  least  distance  at 
which  the  wires  are  safe  from  swinging  together  in  the  middle  of  the 
span;  in  inside  wiring,  by  the  danger  from  fire.  The  other  way  of 
reducing  reactance  is  to  split  the  copper  up  into  a  greater  number  of 
circuits,  and  arrange  these  circuits  so  that  there  is  no  inductive  inter- 
action. For  instance,  suppose  that  in  the  example  worked  out  above, 
two  No.  3  wires  were  used  instead  of  one  No.  0  wire.  The  resistance- 
volts  would  be  practically  the  same,  but  the  reactance-volts  would  be 

244 
less  in  the  ratio  \  X  ~        =  •  535,  since  each  circuit  would  bear  half  the 


current  the  No.  0  circuit  does,  and  the  constant  for  No.  3  wire  i?  .  244, 
instead  of  .  228  —  that  for  No.  0.  The  effect  of  subdividing  the  copper 
is  also  shown  if  in  the  example  given  it  is  desired  to  reduce  the  drop 


50  ELECTRIC  WIRING 

to,  say,  one-half.  Increasing  the  copper  from  No.  0  to  No.  0000  will 
not  produce  the  required  result,  for,  although  the  resistance-volts  will 
be  reduced  one-half,  the  reactance-volts  will  be  reduced  only  in  the 
ratio  .212.  If,  however,  two  inductively  independent  circuits  of  No.  0 

.228* 

wire  be  used,  the  resistance-  and  reactance-volts  will  both  be  reduced 
one-half,  and  the  drop  will  therefore  be  diminished  the  required 
amount. 

The  component  of  drop  due  to  reactance  is  best  diminished  by  sub- 
dividing the  copper  or  by  bringing  the  conductors  closer  together.  It 
is  little  affected  by  change  in  size  of  conductors. 

An  idea  of  the  manner  in  which  changes  of  power  factor  affect 
drop  is  best  gotten  by  an  example.  Assume  distance  of  transmission, 
distance  between  conductors  E.  M.  F.,  and  frequency,  the  same  as  in 
the  previous  example.  Assume  the  apparent  power  delivered  the 
same  as  before,  and  let  it  be  constant,  but  let  the  power  factor  be  given 
several  different  values;  the  true  power  will  therefore  be  a  variable 
depending  upon  the  value  of  the  power  factor.  Let  the  size  of  wire 
be  No.  0000.  As  the  apparent  power,  and  hence  the  current,  is  the 
same  as  before,  and  the  line  resistance  is  one-half,  the  resistance- 
E.  M.  F.  will  in  this  case  be 

15  4 

— — ,  or  7 . 7  per  cent  of  the  E.  M.  F.  delivered. 

Also,  the  reactance-E.  M.  F.  will  be 

.212X17.8      _  . 

228~      -  16. 5  per  cent. 

Combining  these  on  the  chart  for  a  power  factor  of  .4,  and  deducing 
the  drop,  in  per  cent,  of  the  generator  E.  M.  F.,  the  value  obtained  is 
15.3  per  cent;  with  a  power  factor  of  .8,  the  drop  is  14  per  cent; 
with  a  power  factor  of  unity,  it  is  8  per  cenl.  If  in  this  example  the 
true  power,  instead  of  the  apparent  power,  had  been  taken  as  constant, 
it  is  evident  that  the  values  of  drop  would  have  differed  more  widely, 
since  the  current,  and  hence  the  resistance-  and  reactance-volts, 
would  have  increased  as  the  power  factor  diminished.  The  condition 
taken  more  nearly  represents  that  of  practice. 

If  the  line  had  resistance  and  no  reactance,  the  several  values 
of  drop,  instead  of  15.3,  14,  and  8,  would  be  3.2,  5.7,  and  7.2  per 
cent  respectively,  showing  that  for  a  load  of  lamps  the  drop  will  not 


ELECTRIC  WIRING  51 

be  much  increased  by  reactance;  but  that  with  a  load,  such  as  induc- 
tion motors,  whose  power  factor  is  less  than  unity,  care  should  ba 
taken  to  keep  the  reactance  as  low  as  practicable.  In  all  cases  it  is 
advisable  to  place  conductors  as  close  together  as  good  practice  will 
permit. 

When  there  is  a  transformer  in  circuit,  and  it  is  desired  to  obtain 
the  combined  drop  of  transformer  and  line,  it  is  necessary  to  know 
the  resistance-  and  reactance-volts  of  the  transformer.  The  resist- 
ance-volts of  the  combination  of  line  and  transformer  are  the  sum  of 
the  resistance-volts  of  the  line  and  the  resistance-volts  of  the  trans- 
former. Similarly,  the  reactance-volts  of  the  line  and  transformer 
are  the  sum  of  their  respective  reactance-volts.  The  resistance-  and 
reactance-E.  M.  F.s  of  transformers  may  usually  be  obtained  from 
the  makers,  and  are  ordinarily  given  in  per  cent.*  These  per- 
centages express  the  values  of  the  resistance-  and  reactance-E.  M.  F/s 
when  the  transformer  .delivers  its  normal  full-load  current;  and  they 
express  these  values  in  terms  of  the  normal  no-load  E.  M.  F.  of  the 
transformer. 

Consider  a  transformer  built  for  transformation  between  1,000 
and  100  volts.  Suppose  the  resistance-  and  reactance-E.  M.  F.'s  given 
are  2  per  cent  and  7  per  cent  respectively.  Then  the  corresponding 
voltages  when  the  transformer  delivers  full-load  current,  are  2  and  7 
volts  or  20  and  70  volts  according  as  the  line  whose  drop  is  required 
is  connected  to  the  low-voltage  or  high-voltage  terminals.  These 
values,  2 — 7  and  20 — 70,  hold,  no  matter  at  what  voltage  the  trans- 


*When  the  required  values  cannot  be  obtained  from  the  makers,  they  may  be 
measured.  Measure  the  resistance  of  both  coils.  If  the  line  to  be  calculated  is  attached 
to  the  high- voltage  terminals  of  the  transformer,  the  equivalent  resistance  is  that  of  the 
high-voltage  coil,  plus  the  resistance  obtained  by  increasing  in  the  square  of  the  ratio  of 
transformation  the  measured  resistance  of  the  low-voltage  coil  That  is,  if  the  ratio  of 
transformation  is  10,  the  equivalent  resistance  referred  to  the  high-voltage  circuit  is 
the  resistance  of  the  high- voltage  coil,  plus  100  times  that  of  the  low- voltage  coil.  This 
equivalent  resistance  multiplied  by  the  high-voltage  current  gives  the  transformer 
resistance-volts  referred  to  the  high-voltage  circuit.  Similarly,  the  equivalent  resist- 
ance referred  to  the  low- voltage  circuit  is  the  resistance  of  the  low-voltage  coil,  plus  that 
of  the  high- voltage  coil  reduced  in  the  square  of  the  ratio  of  transformation.  It  follows, 
of  course,  from  this,  that  the  values  of  the  resistance- volts  referred  to  the  two  circuits 
bear  to  each  other  the  ratio  of  transformation.  To  obtain  the  reactance-volts,  short- 
circuit  one  coil  of  the  transformer  arid  measure  the  voltage  necessary  to  force  through 
the  other  coil  its  normal  current  at  normal  frequency.  The  result  is,  nearly  enough, 
the  reactance-volts.  It  makes  no  difference  which  coil  is  short-circuited,  as  the  results 
obtained  in  one  case  will  bear  to  those  in  the  other  the  ratio  of  transformation.  If  a 
close  value  is  desired,  subtract  from  the  square  of  the  voltage  reading  the  square  of  the 
resistance-volts,  and  take  the  square  root  of  the  difference  as  the  reactance- volts. 


52  ELECTRIC  WIRING 

former  is  operated,  since  they  depend  only  upon  the  strength  of  cur- 
rent, providing  it  is  of  the  normal  frequency.  If  any  other  than  the 
full-load  current  is  drawn  from  the  transformer,  the  reactance-  and 
resistance-volts  will  be  such  a  proportion  of  the  values  given  above 
as  the  current  flowing  is  of  the  full-load  current.  It  may  be  noted,  in 
passing,  that  when  the  resistance-  and  reactance-volts  of  a  trans- 
former are  known,  its  regulation  may  be  determined  by  making  use 
of  the  chart  in  the  same  way  as  for  a  line  having  resistance  and 
reactance. 

As  an  illustration  of  the  method  of  calculating  the  drop  in  a 
line  and  transformer,  and  also  of  the  use  of  table  and  chart  in  calculat- 
ing low-voltage  mains,  the  following  example  is  given  : 

Problem.  A  single-phase  induction  motor  is  to  be  supplied  with  20  am- 
peres at  200  volts;  alternations,  7,200  per  minute;  power  factor,  .78.  The 
distance  from  transformer  to  motor  is  150  feet,  and  the  line  is  No.  5  wire,  6 
inches  between  centers  of  conductors.  The  transformer  reduces  in  the  ratio 

2  000 
'        ,  has  a  capacity  of  25  amperes  at  200  volts,  and,  when  delivering  this 

current  and  voltage,  its  re&Istance-E.  M.  F.  is  2.5  per  cent,  its  reactance- 
E.  M.  F.  5  per  cent.  Find  the  drop. 

The  reactance  of  1,000  feet  of  circuit  consisting  of  two  No.  5 
wires,  6  inches  apart,  is  .204.  The  reactance-volts  therefore  are 


.204  X         ~  X  20  =  .61  volts. 
1  ,uUU 

The  resistance-volts  are 

.627  X  r        X  20  =  1.88  volts. 


At  25  amperes,  the  resistance-volts  of  the  transformer  are  2.5  per 

20 

cent  of  200,  or  5  volts.    At  20  amperes,  they  are  —  of  this,  or  4  volts. 

«u 

Similarly,  the  transformer  reactance-volts  at  25  amperes  are  10, 
and  at  20  amperes  are  8  volts.  The  combined  reactance-volts  of 
transformer  and  line  are  8  +  .61  =  8.61,  which  is  4.3  per  cent  of 
the  200  volts  to  be  delivered.  The  combined  resistance-volts  are  1.88 
+4,  or  5.88,  which  is  2.94  per  cent  of  the  E.  M.  F.  to  be  delivered. 
Combining  these  quantities  on  the  chart  with  a  power  factor  of  .78, 
the  drop  is  5  per  cent  of  the  delivered  E.  M.  F., 

or  -—  =  4.8  per  cent 
105 

of  the  impressed  E.  M.  F.    The  transformer  must  be  supplied  with 


ELECTRIC  WIRING  53 


=  2,100  volts, 

in  order  that  200  volts  shall  be  delivered  to  the  motor. 

Table  X  (page  47)  is  made  out  for  7,200  alternations,  but  will 
answer  for  any  other  number  if  the  values  for  reactance  be  changed 
in  direct  proportion  to  the  change  in  alternations.  For  instance, 

i  f\  nnn 

for   16,000   alternations,   multiply  the   reactances  given   by       '       • 

•  ,^UU 

For  other  distances  between  centers  of  conductors,  interpolate  the 
values  given  in  the  table.  As  the  reactance  values  for  different  sizes 
of  wire  change  by  a  constant  amount,  the  table  can,  if  desired,  be 
readily  extended  for  larger  or  smaller  conductors. 

The  table  is  based  on  the  assumption  of  sine  currents  and 
E.  M.  F.'s.  The  best  practice  of  to-day  produces  machines  which 
so  closely  approximate  this  condition  that  results  obtained  by  the 
above  methods  are  well  within  the  limits  of  practical  requirements. 

Polyphase  Circuits.  So  far,  single-phase  circuits  only  have 
been  dealt  with.  A  simple  extension  of  the  methods  given  above 
adapts  them  to  the  calculation  of  polyphase  circuits.  A  four-wire 
quarter-phase  (two-phase)  transmission  may,  so  far  as  loss  and  regula- 
tion are  concerned,  be  replaced  by  two  single-phase  circuits  identical 
(as  to  size  of  wire,  distance  between  wires,  current,  and  E.  M.  F.) 
with  the  two  circuits  of  the  quarter-phase  transmission,  provided  that 
in  both  cases  there  is  no  inductive  interaction  between  circuits.  There- 
fore, to  calculate  a  four-wire,  quarter-phase  transmission,  compute 
the  single-phase  circuit  required  to  transmit  one-half  the  power  at 
the  same  voltage.  The  quarter-phase  transmission  will  require  two 
such  circuits. 

A  three-wire,  three-phase  transmission,  of  which  the  conductors 
are  symmetrically  related,  may,  so  far  as  loss  and  regulation  are 
concerned,  be  replaced  by  two  single-phase  circuits  having  no  in- 
ductive interaction,  and  identical  with  the  three-phase  line  as  to 
size,  wire,  and  distance  between  wires.  Therefore,  to  calculate  a 
three-phase  transmission,  calculate  a  single-phase  circuit  to  ca^ry 
one-half  the  load  at  the  same  voltage.  The  three-phase  transmis- 
sion will  require  three  wires  of  the  size  and  distance  between  centers 
as  obtained  for  the  single-phase. 

SL   three-wire,    two-phase     transmission     may    be    calculated 


54  ELECTRIC  WIRING 

exactly  as  regards  loss,  and  approximately  as  regards  drop,  in  the 
same  way  as  for  three-phase.  It  is  possible  to  exactly  calculate 
the  drop,  but  this  involves  a  more  complicated  method  than  the 
approximate  one.  The  error  by  this  approximate  method  is  gen- 
erally small.  It  is  possible,  also,  to  get  a  somewhat  less  drop  and 
loss  with  the  same  copper  by  proportioning  the  cross-section  of 
the  middle  and  outside  wires  of  a  three-wire,  quarter-phase  circuit 
to  the  currents  they  carry,  instead  of  using  three  wires  of  the  same 
size.  The  advantage,  of  course,  is  not  great,  and  it  will  not  be  con- 
sidered here. 

WIRING  AN  OFFICE  BUILDING 

The  building  selected  as  a  typical  sample  of  a  wiring  installation 
is  that  of  an  office  building  located  in  Washington,  D.  C.  The  figures 
shown  are  reproductions  of  the  plans  actually  used  in  installing  the 
work. 

The  building  consists  of  a  basement  and  ten  stories.  It  is  of 
fireproof  construction,  having  steel  beams  with  terra-cotta  flat  arches. 
The  main  walls  are  of  brick  and  the  partition  walls  of  terra-cotta 
blocks,  finished  with  plaster.  There  is  a  space  of  approximately  five 
inches  between  the  top  of  the  iron  beams  and  the  top  of  the  finished 
floor,  of  which  space  about  three  inches  was  available  for  running 
the  electric  conduits.  The  flooring  is  of  wood  in  the  offices,  but  of 
concrete,  mosaic,  or  tile  in  the  basement,  halls,  toilet-rooms, 
etc. 

The  electric  current  supply  is  derived  from  the  mains  of  the  local 
illuminating  company,  the  mains  being  brought  into  the  front  of  the 
building  and  extending  to  a  switchboard  located  near  the  center  of  the 
basement. 

As  the  building  is  a  very  substantial  fireproof  structure,  the  only 
method  of  wiring  considered  was  that  in  which  the  circuits  would  be 
installed  in  iron  conduits. 

Electric  Current  Supply.  The  electric  current  supply  is  direct 
current,  two-wire  for  power,  and  three-wire  for  lighting,  having  a 
potential  of  236  volts  between  the  outside  conductors,  and  118  volte 
between  the  neutral  and  either  outside  conductor. 


ELECTRIC  WIRING  55 

Switchboard.  On  the  switchboard  in  the  basement  are  mounted 
wattmeters,  provided  by  the  local  electric  company,  and  the  various 
switches  required  for  the  control  and  operation  of  the  lighting  and 
power  feeders.  There  are  a  total  of  ten  triple-pole  switches  for  light- 
ing, and  eighteen  for  power.  An  indicating  voltmeter  and  ampere 
meter  are  also  placed  in  the  switchboard.  A  voltmeter  is  provided 
with  a  double-throw  switch,  and  so  arranged  as  to  measure  the  poten- 
tial across  the  two  outside  conductors,  or  between  the  neutral  con- 
ductor and  either  of  the  outside  conductors.  The  ampere  meter  is 
arranged  with  two  shunts,  one  being  placed  in  each  outside  leg;  the 
shunts  are  connected  with  a  double-pole,  double-throw  switch,  so 
that  the  ampere  meter  can  be  connected  to  either  shunt  and  thus 
measure  the  current  supplied  on  each  side  of  the  system. 

Character  of  Load.  The  building  is  occupied  partly  as  a  news- 
paper office,  and  there  are  several  large  presses  in  addition  to  the  usual 
linotype  machines,  trimmers,  shavers,  cutters,  saws,  etc.  There  are 
also  electrically-driven  exhaust  fans,  house  pumps,  air-compressors, 
etc.  The  upper  portion  of  the  building  is  almost  entirely  devoted 
to  offices  rented  to  outside  parties.  The  total  number  of  motors 
supplied  was  55;  and  the  total  number  of  outlets,  1,100,  supplying 
2,400  incandescent  lamps  and  4  arc  lamps. 

Feeders  and  Mains.  The  arrangement  of  the  various  feeders 
and  mains,  the  cut-out  centers,  mains,  etc.,  which  they  supply,  are 
shown  diagrammatically  in  Fig.  41,  which  also  gives  in  schedule  the 
sizes  of  feeders,  mains,  and  motor  circuits,  and  the  data  relating  to  the 
cut-out  panels. 

Although  the  current  supply  was  to  be  taken  from  an  outside 
source,  yet,  inasmuch  as  there  was  a  probability  of  a  plant  being  in- 
stalled in  the  building  itself  at  some  future  time,  the  three-wire  system 
of  feeders  and  mains  was  designed,  with  a  neutral  conductor  equal 
to  the  combined  capacity  of  the  two  outside  conductors,  so  that 
120-volt  two-wire  generators  could  be  utilized  without  any  change  in 
the  feeders. 

Basement.  The  plan  of  the  basement,  Fig.  42,  shows  the  branch 
circuit  wiring  for  the  outlets  in  the  basement,  and  the  location  of  the 
main  switchboard.  It  also  shows  the  trunk  cables  for  the  inter- 
connection system  serving  to  provide  the  necessary  wires  for  telephones, 


FEEDERS 


X  ALL  CONDUCTORS  IN  ONE  CONDUIT. 
XX  SEPARATE  CONDUIT  FOR  E  ACH  CONDUCTOR 
XXX  THIS  FEEDER  IS  TO  BE  DIVIDED  INTO  FOUR  (-*) 
»     CONDUCTORS  OF  H  2OOOOOO  CM.  EACH. 
"     EACH  CONDUCTOR  IS  TO  BE  INSTALLED  IN  A  SEP- 

-   ARATE  3"  (INSIDE  DIAM)  CONDUIT 
I_S?- LIGHTING  SUPPLY 

P.S.=POWER 

XXX*  SEPARATE  3" (INSIDE  DIAt^CONDUIT  FOR  EACH 
••      CONDUCTOR 


X  ALL  CONDUCTORS  IN  ONE  CONDUIT 

MOTOR  CIRCUITS 


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Fig.  41.    Wiring  of  an  Office  Building.    Diagram  Showing  Arrangement  of 
Feeders  and  Mains,  Cut-Out  Centers,  etc. 


ELECTRIC  WIRING 


57 


SCHEDULE    OF     CIRCUITS' 


CABt_E    NO.  &      RISES  TO    S^d  FLOOR 


Showing    Explanation 
Of  Various    Symbols    used  in 
Figs.  41  to  46  Inclusive 


Ceiling  Chandelier 
Wall  Bracket 
—--Gooseneck  Bracket 
Wall  Socket 
Drop  Cord 
Arc  Lamp 
Cooper-Hewitt  Lamp 
Cluster 
Floor  Outlet 


—-Desk  Light 

Extension  Outlet 


Push  Button  Switch 

Snap  Swltchi 

-Junction  Box 
—Electric  Clock 

Master  Clock 

Motor    Starter 

Cut-Out   Panel . 

Interconnection  Box 

Power   Panel 

-Pull    Box 

Circuit  under  Floor 

"          "  "     above 

"'     on  Ceiling-  Exposed 

Service  Line  under  Floor 


Fig.  42.    Wiring  an  Office  Building.    Basement  Plan  Showing  Branch  Circuit  Wiring  for 

Outlets  in  Basement,  Location  of  Main  Switchboard,  and  Trunk  Cables  of  the 

Interconnection  System  Providing  Wires  for  Telephone, 

Ticker,  and  Messenger  Call  Service,  etc, 


58  ELECTRIC  WIRING 

tickers,  messenger  calls,  etc.,  in  all  the  rooms  throughout  the  building, 
as  will  be  described  later. 

To  avoid  confusion,  the  feeders  were  not  shown  on  the  basement 
plan,  but  were  described  in  detail  in  the  specification,  and  installed 
in  accordance  with  directions  issued  at  the  time  of  installation.  The 
electric  current  supply  enters  the  building  at  the  front,  and  a  service 
switch  and  cut-out  are  placed  on  the  front  wall.  From  this  point,  a 
two-wire  feeder  for  power  and  a  three-wire  feeder  for  lighting,  are 
run  to  the  main  switchboard  located  near  the  center  of  the  basement. 
Owing  to  the  size  of  the  conduits  required  for  these  supply  feeders,  as 
well  as  the  main  feeders  extending  to  the  upper  floors  of  the  building, 
the  said  conduits  are  run  exposed  on  substantial  hangers  suspended 
from  the  basement  ceiling. 

First  Floor.  The  rear  portion  of  the  building  from  the  basement 
through  the  first  floor,  Fig.  43,  and  including  the  mezzanine  floor, 
between  the  first  and  second  floors,  at  the  rear  portion  of  the  building 
only,  is  utilized  as  a  press  room  for  several  large  and  heavy,  modern 
newspaper  presses.  The  motors  and  controllers  for  these  presses  are 
located  on  the  first  floor.  A  separate  feeder  for  each  of  these  press 
motors  is  run  directly  from  the  main  switchboard  to  the  motor  con- 
troller in  each  case.  Empty  conduits  were  provided,  extending  from 
the  controllers  to  the  motor  in  each  case,  intended  for  the  various 
control  wires  installed  by  the  contractor  for  the  press  equipments. 

One-half  of  the  front  portion  of  the  first  floor  is  utilized  as  a  news- 
paper office;  the  remaining  half,  as  a  bank. 

Second  Floor.  The  rear  portion  of  the  second  floor,  Fig.  44,  is 
occupied  as  a  composing  and  linotype  room,  and  is  illuminated  chiefly 
by  means  of  drop-cords  from  outlets  located  over  the  linotype  machines 
and  over  the  compositors'  cases.  Separate  ^-horse-power  motors 
are  provided  for  each  linotype  machine,  the  circuits  for  the  same  being 
run  underneath  the  floor. 

Upper  Floors.  A  typical  plan  (Fig.  45)  is  shown  of  the  upper 
floors,  as  they  are  similar  in  all  respects  with  the  exception  of  certain 
changes  in  partitions,  which  are  not  material  for  the  purpose  of  illus- 
tration or  for  practical  example.  The  circuit  work  is  sufficiently 
intelligible  from  the  plan  to  require  no  further  explanation. 

Interconnection  System.  Fig.  46  is  a  diagram  of  the  intercon- 
nection system,  showing  the  main  interconnection  box  located  in  the 


ELECTRIC  WIRING 


59 


SCHEDULE   OF    CIRCUITS 


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WALL  BETWEEN 
PRESS  ROOM  & 
PAPER  STORAGE 


PRESS  R'M 


SKETCH    SHOWING    ROUTE     OF 

EMPTY    CONDUITS     RISIN3    TO    COSH 

TROLLER     PLATFORM    IN    PRESS 

ROOM.. 


NOTE:  ALL     CONDUITS    FOR   THE    PRESSES     MUST    COME 

THROUGH    THE   CONTROLLER     PLATFORM    2 1"  FROM 
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ALL   CONDUITS  ARE;  TO  TERMINATE  IN  PRESS  Aia 

•DIRECTE.P. 


Fig.  43.    Wiring  of  an  Office  Building. 

First-Floor  Plan,  Showing  Press  Room  in  Rear,  Containing  Motors  and  Controllers  for 

Newspaper  Presses,  Fed  Directly  from  Main  Switchboard  in  Basement; 

also,  in  front,  Newspaper  Counting  Room  and  Bank  Offices. 


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Fig.  44.    Wiring  of  an  Office  Building.    Plan  of  Second  Floor.    Rear  Portion  Occupied  a» 
a  Composing  and  Linotype  Room. 


ELECTRIC  WIRING 


61 


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Fig.  45.    Wiring  of  an  Office  Building. 

Typical  Plan  of  Upper  Floors,  Showing  Circuit  Work,  Schedule,  etc.  All  the  Floors  above 

the  Second  are  Similar  to  One  Another  in  Plan,  Differing  Only  in  Comparatively 

Unimportant  Details  of  Partitions. 


—  CABLES.— 


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Fig.  46.    Wiring  of  an  Office  Building.     Diagram  of  the  Interconnection  System. 


ELECTRIC  WIRING  63 

basement;  adjoining  this  main  box  is  located  the  terminal  box  of  the 
local  telephone  company.  A  separate  system  of  feeders  is  provided 
for  the  ticker  system,  as  these  conductors  require  somewhat  heavier 
insulation,  and  it  was  thought  inadvisable  to  place  them  in  the  same 
conduits  with  the  telephone  wires,  owing  to  the  higher  potential  of 
ticker  circuits.  A  separate  interconnection  cable  runs  to  each  floor, 
for  telephone  and  messenger  call  purposes;  and  a  central  box  is  placed 
near  the  rising  point  at  each  floor,  from  which  run  subsidiary  cables 
to  several  points  symmetrically  located  on  the  various  floors.  From 
these  subsidiary  boxes,  wires  can  be  run  to  the  various  offices  requiring 
telephone  or  other  service.  Small  pipes  are  provided  to  serve  as  race- 
ways from  office  to  office,  so  as  to  avoid  cutting  partitions.  In  this 
way,  wires  can  be  quickly  provided  for  any  office  in  the  building  with- 
out damaging  the  building  in  any  way  whatever;  and,  as  provision  is 
made  for  a  special  wooden  moulding  near  the  ceiling  to  accommodate 
these  wires,  they  can  be  run  around  the  room  without  disfiguring  the 
walls.  All  the  main  cables  and  subsidiary  wires  are  connected  with 
special  interconnection  blocks  numbered  serially;  and  a  schedule  is 
provided  in  the  main  interconnection  box  in  the  basement,  which 
enables  any  wire  originating  thereat,  to  be  readily  and  conveniently 
traced  throughout  the  building.  All  the  main  cables  and  subsidiary 
cables  are  run  in  iron  conduits. 

OUTLET=BOXES,  CUT-OUJ  PANELS,  AND 
OTHER  ACCESSORIES 

Outlet-Boxes.  Before  the  introduction  of  iron  conduits,  outlet- 
boxes  were  considered  unnecessary,  and  with  a  few  exceptions  were 
not  used,  the  conduits  being  brought  to  the  outlet  and  cut  off  after  the 
walls  and  ceilings  were  plastered.  With  the  introduction  of  iron  con- 
duits, however,  the  necessity  for  outlet-boxes  was  realized;  and  the 
Rules  of  the  Fire  Underwriters  were  modified  so  as  to  require  their  use. 
The  Rules  of  the  National  Electric  Code  now  require  outlet-boxes  to 
be  used  with  rigid  iron  and  flexible  steel  conduits,  and  with  armored 
cables.  A  portion  of  the  rule  requiring  their  use  is  as  follows : 

All  interior  conduits  and  armored  cables  "must  be  equipped  at  every 
outlet  with  an  approved  outlet-box  or  plate. 

"Outlet-plates  must  not  be  used  where  it  is  practicable  to  install  outlet- 
boxes. 


64 


ELECTRIC  WIRING 


"In  buildings  already  constructed,  where  the  conditions  are  such  that 
neither  outlet-box  nor  plate  can  be  installed,  these  appliances  may  be  omitted 
by  special  permission  of  the  inspection  department  having  jurisdiction,  pro- 
viding the  conduit  ends  are  bushed  and  secured." 

Fig.  47  shows  a  typical  form  of  outlet-box  for  bracket  or  ceiling 
outlets  of  the  universal  type.  When  it  is  desired  to  make  an  opening 
for  the  conduits,  a  blow  from  a  hammer 
will  remove  any  of  the  weakened  portion 
of  the  wall  of  the  outlet-box,  as  may  be  re- 
quired. This  form  of  outlet-box  is  fre- 
quently referred  to  as  the  knock-out  type. 
Other  forms  of  outlet-boxes  are  made  with 
the  openings  cast  in  the  box  at  the  re- 
quired points,  this  class  being  usually 
stronger  and  better  made  than  the  univer- 
sal type.  The  advantages  of  the  universal 
type  of  outlet-box  are  that  one  form  of  box  will  serve  for  any  ordinary 
conditions,  the  openings  being  made  according  to  the  number  of 
conduits  and  the  directions  in  which  they  enter  the  box. 

Fig.  48  shows  a  waterproof  form  of  outlet-box  used  out  of  doors, 
or  in  other  places  where  the  conditions  require  the  use  of  a  water- 
tight and  waterproof  outlet-box. 

It  will  be  seen  in  this  case,  that  the  box  is  threaded  for  the  con- 


Fig.  47.    Universal  and 

Knock-Out  Type  of 

Outlet  Box. 


OD 


Fig.  48.     Water-Tight  Outlet  Box. 
Courtesy  of  H.  Krantz  Manufacturing  Co.,  Brooklyn,  N.  1 . 

duits,  and  that  the  cover  is  screwed  on  tightly  and  a  flange  provided 
for  a  rubber  gasket. 


ELECTRIC  WIRING 


65 


Figs.  49  and  50  show  water-tight  floor  boxes  which  are  for  outlets 
located  in  the  floor.  While  the  rules  do  not  require  that  the  floor  outlet- 
box  shall  be  water-tight,  it  is  strongly  recommended  that  a  water- 
tight outlet  be  used  in  all  cases  for  floor  connections.  In  this  case 
also,  the  conduit  opening  is  threaded,  as  well  as  the  stem  cover  through 
which  the  extension  is  made  in  the  conduit  to  the  desk  or  table.  When 
the  floor  outlet  connection  is  not  required,  the  stem  cover  may  be 
removed  and  a  flat,  blank  cover  be  used  to  replace  the  same. 

A  form  of  outlet-box  used  for  flexible  steel  cables  and  steel  ar- 
mored cable,  has  already  been  shown  (see  Fig.  5). 

There  is  hardly  any  limit  to  the  number  and  variety  of  makes  of 
outlet-boxes  on  the  market,  adapted  for  ordinary  and  for  special  con- 


Fig.  49. 


Types  of  Floor  Outlet-Boxes. 


Fig.  50. 


ditions;  but  the  types  illustrated  in  these  pages  are  characteristic  and 
typical  forms. 

Bushings.  The  Rules  of  the  National  Electric  Code  require  that 
conduits  entering  junction-boxes,  outlet-boxes,  or  cut-out  cabinets 
shall  be  provided  with  approved  bushings,  fitted  to  protect  the  wire 
from  abrasion. 

Fig.  51  shows  a  typical  form  of  conduit  bushing.  This  bushing 
is  screwed  on  the  end  of  the  conduit  after  the  latter  has  been  intro- 
duced into  the  outlet-box,  cut-out  cabinet,  etc.,  thereby  forming  an 
insulatec[  orifice  to  protect  the  wire  at  the  point  where  it  leaves  the 
conduits,  and  to  prevent  abrasion,  grounds,  short  circuits,  etc.  A 
lock-nut  (Fig.  52)  is  screwed  on  the  threaded  end  of  the  conduit  before 
the  conduit  is  placed  in  the  outlet-box  or  cut-out  cabinet,  and  this 
lock-nut  and  bushing  clamp  the  ronduH  securely  in  position.  Fig. 


66 


ELECTRIC  WIRING 


Fig.  51.    Conduit  Bushing. 


53  shows  a  terminal  bushing  for  panel-boxes  used  for  flexible  steel 
conduit  or  armored  cable. 

The  Rules  of  the  National  Electric  Code  require  that  the  metal 
of  conduits  shall  be  permanently  and  effectually  grounded,  so  as  to 

insure  a  positive 
connection  for 
grounds  or  leak- 
ing currents,  and 
in  order  to  pro- 
vide a  path  of 
least  resistance 
to  prevent  the 
current  from 
finding  a  path 

through  any  source  which  might  cause  a  fire.  At  outlet-boxes,  the 
conduits  and  gaspipes  must  be  fastened  in  such  a  manner  as  to 
insure  good  electrical  connection;  and  at  centers  of  distribution, 
the  conduits  should  be  joined  by  suitable  bond 
wires,  preferably  of  copper,  the  said  bond  wires 
being  connected  to  the  metal  structure  of  the 
building,  or,  in  case  of  a  building  not  having 
an  iron  or  steel  structure,  being  grounded  in  a 
permanent  manner  to  water  or  gas  piping. 
Fuse-Boxes,  Cut-Out  Panels,  etc.  From  the  very  outset,  the 
necessity  was  apparent  of  having  a  protective  device  in  circuit  with 
the  conductor  to  protect  it  from  overload,  short  circuits,  etc.  For 
this  purpose,  a  fusible 
metal  having  a  low 
melting  point  was  em- 
ployed. The  form  of 
this  fuse  has  varied 
greatly.  Fig.  54  shows 

ohar^ctpristiV    form 

of  what  is  known  as 

the  link  fuse  with  copper  terminals,  on  which  are  stamped  the  ca- 

pacity of  the  fuse. 

The  form  of  fuse  used  probably  to  a  greater  extent  than  any  other, 
although  it  is  now  being  superseded  by  other  more  modern  forms, 


Fig.  52.    Lock-Nut. 


Fig.  53.    Panel-Box  Terminal  Bushing. 
Courtesy  of  Sprague  Electric  Co.,  New  York,  N.  1. 


ELECTRIC  WIRING 


67 


is  that  known  as  the  Edison  fuse-plug,  shown  in  Fig.  55.     A  porcelain 
cut-out  block  used  with  the  Edison  fuse  is  shown  in  Fig.  56. 

Within  the  last  four  or  five  years,  a  new  form  of  fuse,  known  as 
the  enclosed  fuse,  has  been  introduced  and  used  to  a    considerable 


Fig.  54.    Copper-Tipped  Fuse  Link. 


Fig.  55.    Edison  Fuse-Plug, 
Courtesy  of 'General Electric  Co., Schenectady.N.  Y. 


Fig.  56.  Porcelain  Cut-Out  Block. 

Courtesy  ofGeneralElectricCo., 

Schenectady,  Jf.  Y. 


extent.  A  fuse  of  this  type  is  shown  in  Fig.  57.  Fig.  58  gives  a  sec- 
tional view  of  this  fuse,  showing  the  porous  filling  surrounding  the 
fuse-strips,  and  also  the  device  for  indicating  when  the  fuse  has 
blown.  This  form  of  fuse  is  made  with  various  kinds  of  terminals; 

it  can  be  used  with  spring  clips  in  small 
sizes,  and  with  a  post  screw  contact  in 
larger  sizes.  For  ordinary  low  potentials 
this  fuse  is  desirable  for  currents  up  to 
25  amperes;  but  it  is  a  debatable  ques- 
tion whether  it  is  desirable  to  use  an  en- 
closed fuse  for  heavier  currents.  Fig.  59 
shows  a  cut-out  box  with  Edison  plug 
fuse-blocks  used  with  knob  and  tube  wiring.  It  will  be  seen  that 
there  is  no  connection  compartment  in  this  fuse-box,  as  the  circuits 
enter  directly  opposite  the  terminals  with  which  they  connect. 

Fig.  60  shows  a  cut-out  panel  adapted  for  enclosed  fuses,  and 
installed  in  a  cab- 
inet having  a  con- 
nection  compart- 
ment. As  will  be 
seen  from  the  cut, 
the  tablet  itself  is 
surrounded  on  the 

Fig.  58.    Section  of  Enclosed  Fuse. 

four  sides  by  slate, 

which  is  secured  in  the  corners  by  angle-irons.  The  outer  box  may 
be  of  wood  lined  with  sheet  iron,  or  it  may  be  of  iron.  Fig.  61 
shows  a  door  and  trim  for  a  cabinet  of  this  type.  It  will  be  seen  that 


Fig.  57.    Enclosed  or  "Cartridge"  Fuse. 


ELECTRIC  WIRING 


i  •  i 


the  door  opens  only  on  the  center  panel,  and  that  the  trim  covers  and 
conceals  the  connection  compartment.  The  inner  side  of  the  door 
should  be  lined  with  slate,  and  the  inner  side  of  the  trim  should  be 
lined  with  sheet  iron.  Fig.  62  shows  a  sectional  view  of  the  cabinet 
and  panel.  In  this  type  of  cabinet,  the  conduits  may  enter  at  any 

point,  the  wires  being 
run  to  the  proper  con- 
nectors in  the  connection 
compartment. 

Figs.  63  and  64  illus- 
trate a  type  of  panel- 
board  and  cabinet  hav- 
ing a  push-button  switch 
connected  with  each 
branch  circuit  and  so 
arranged  that  the  cut- 
out panel  itself  may  be 
enclosed  by  locked  doors, 
and  access  to  the  switches 
may  be  obtained  through 
two  separate  doors  pro- 
vided with  latches  only. 
This  type  of  panel  was  arranged  and  designed  by  the  author  of  this 
instruction  paper. 

OVERHEAD  LINEWORK 

The  advantages  of  overhead  linework  as  compared  with  under- 
ground linework  are  that  it  is  much  less  expensive;  it  is  more  readily 
and  more  quickly  installed ;  and  it  can  be  more  readily  inspected  and 
repaired. 

Its  principal  disadvantages  are  that  it  is  not  so  permanent  as 
underground  linework;  it  is  more  easily  deranged;  and  it  is  more 
unsightly. 

For  large  cities,  and  in  congested  districts,  overhead  linework 
should  not  be  used.  However,  the  question  of  first  cost,  the  question 
of  permanence,  and  the  municipal  regulations,  are  usually  the  factors 
which  determine  whether  overhead  or  underground  linework  shall 
be  used. 


•    : 


Fig.  59.    Porcelain  Cut-Outs  in  Wooden  Box. 
Courtesy  of  H.  T.  Paiste  Co.,  Philadelphia,  Pa. 


ELECTRIC  WIRING 


The  principal  factors  to  be  considered  in  overhead  linework  will 
be  briefly  outlined. 

Placing  of  Poles.  As  a  general  rule,  the  poles  should  be  set  from 
100  to  125  feet  apart,  which  is  equivalent  to  53  to  42  poles  per  mile. 
Under  certain  conditions,  these  spacings  given  will  have  to  be  modified ; 
but  if  the  poles  are  spaced  too  far  apart,  there  is  danger  of  too  great 
a  strain  on  the  poles  themselves,  and  on  the  cross-arms,  pins,  and 


Fig.  60.     Plan  View,  Cover,  and 
Section  of  Double  Cut-Out  Box.  Fig.  61. 

conductors.  If,  on  the  other  hand, 
they  are  placed  too  close  together, 
the  cost  is  unnecessarily  increased. 
The  size  and  number  of  conduct- 
ors, and  the  potential  of  the  line-  Fig.  62. 
work,  determine  to  a  great  extent 

the  distance  between  the  poles;  the  smaller  the  size,  the  less  the  num- 
ber of  conductors;  and  the  lower  the  potential,  the  greater  the  distance 
between  the  poles  may  be  made.  Of  course,  the  exact  location  of 
the  poles  is  subject  to  variation  because  of  trees,  buildings,  or  other 
obstructions.  The  usual  method  employed  in  locating  poles,  is  first 
to  make  a  map  on  a  fairly  large  scale,  showing  the  course  of  the  line- 
work,  and  then  to  locate  the  poles  en  the  ground  according  tc  the  actual 
conditions. 


70  ELECTRIC  WIRING 

Poles.  Poles  should  be  of  selected  quality  of  chestnut  or  cedar, 
and  should  be  sound  and  free  from  cracks,  knots,  or  other  flaws. 
Experience  has  proven  that  chestnut  and  cedar  poles  are  the  most 
durable  and  best  fitted  for  linework.  If  neither  chestnut  nor  cedar 
poles  can  be  obtained,  northern  pine  may  be  used,  and  even  other 
timber  in  localities  where  these  poles  cannot  be  obtained;  but  it  is 
found  that  the  other  woods  do  not  last  so  long  as  those  mentioned, 


Pig.  63.   Cut-Otit  Panel  with  Push-Button  Switches.   Cover  Removed. 

and  some  of  the  other  woods  are  not  only  less  strong  initially,  but  are 
apt  to  rot  much  quicker  at  the  "wind  and  water  line" — that  is,  just 
above  and  below  the  surface  of  the  ground. 

The  proper  height  of  pole  to  be  used  depends  upon  conditions. 
In  country  and  suburban  districts,  a  pole  of  25  to  30  feet  is  usually 
of  sufficient  height,  unless  there  are  more  than  two  or  three  cross-arms 
required.  In  more  densely  populated  districts  and  in  cities  where  a 
great  number  of  cross-arms  are  required,  the  poles  may  have  to  be 


ELECTRIC  WIRING 


71 


40  to  60  feet,  or  even  longer.  Of  course,  the  longer  the  pole,  the 
greater  the  possibility  of  its  breaking  or  bending;  and  as  the  length 
increases,  the  diameter  of  the  butt  end  of  pole  should  also  increase. 
Table  XI  gives  the  average  diameters  required  for  various  heights  of 
poles,  and  the  depth  the  poles  should  be  placed  in  the  ground.  These 
data  have  been  compiled  from  a  number  of  standard  specifications. 

TABLE  XI 

Pole  Data 


LENGTH  OF  POLE 

DIAMETER  6  IN. 
FROM  BUTT 

DIAMETER  AT  Top 

DEPTH  POLE  SHOULD 
BE  PLACED  IN 
GROUND 

25  feet 

9  to  10  in. 

6  to  8  in. 

5  feet 

30 

11 

5^ 

35 

12 

5| 

40 

13 

6 

45 

14 

6^ 

50 

15 

7 

55 

16  to  17 

7£ 

60 

18 

7* 

65 

19 

8 

70 

20 

8 

75 

21 

8£ 

80 

22 

9 

As  it  is  somewhat  difficult,  because  of  irregularities  in  size,  to  measure  the  diame- 
ter of  some  poles,  the  circumference  may  be  measured  instead:  then,  by  multiplying 
the  diameters  given  in  the  above  table,  by  3,1416,  the  measurements  may  be  reduced 
to  the  circumference  in  inches. 

The  minimum  diameters  of  the  pole  at  the  top,  which  should  be 
allowed,  will  depend  largely  on  the  size  of  the  conductors  used,  and 
on  the  potential  carried  by  the  circuits;  the  larger  the  conductors 
and  the  higher  the  potentials,  the  greater  should  be  the  diameter  at 
the  top  of  the  pole. 

Poles  should  be  shaved,  housed,  and  -gained,  also  cleaned  and 
ready  for  painting,  before  erection. 

Poles  should  usually  be  painted,  not  only  for  the  sake  of  appear- 
ance, but  also  in  order  to  preserve  them  from  the  weather.  It  is  par- 
ticularly important  that  they  should  be  protected  at  their  butt  end,  not 
only  where  they  are  surrounded  by  the  ground,  but  for  a  foot  or  two 
above  the  ground,  as  it  is  at  this  point  that  poles  usually  deteriorate 
most  rapidly.  Painting  is  not  so  satisfactory  at  this  point  as  the  us6 
of  tar,  pitch,  or  creosote.  The  life  of  the  pole  can  be  increased  con« 
siderably  by  treating  it  with  one  or  another  of  these  preservatives. 


72 


ELECTRIC  WIRING 


Before  any  poles  are  erected,  they  should  be  closely  inspected  for 
flaws  and  for  crookedness  or  too  great  departure  from  a  straight  line. 
Where  appearance  is  of  considerable  importance,  octagonal  poles 
may  be  used,  although  these  cost  considerably  more  than  round  poles. 
Gains  or  notches  for  the  cross-arms  should  be  cut  in  the  poles  before 
they  are  erected,  and  should  be  cut  square  with  the  axis  of  the  pole, 
and  so  that  the  cross-arms  will  fit  snugly  and  tightly  within  the  space 
thus  provided.  These  gains  should  be  not  less  than  4J  inches  wide, 


Fig.  64.    Cut-Out  Panel  with  Push-Button  Switches.    With  Cover. 

nor  less  than  J  inch  deep.  Gains  should  not  be  placed  closer  than  24 
inches  between  centers,  and  the  top  gains  should  be  at  least  9  inches 
from  the  apex  of  the  pole. 

Pole  Guying.  Where  poles  are  subject  to  peculiar  strains  due 
to  unusual  stress  of  the  wires,  such  as  at  corners,  etc.,  guys  should  be 
employed  to  counteract  the  strain  and  to  prevent  the  pole  from  being 
bent  and  finally  broken,  or  from  being  pulled  from  its  proper  position. 


ELECTRIC  WIRING 


73 


Where  there  are  a  consider- 
able number  of  wires  on  the  poles, 
or  in  case  of  unusually  long 
poles,  or  where  the  linework  is 
subject  to  severe  storms,  it  is 
frequently  necessary  to  guy  the 
poles  even  on  straight  linework. 
In  such  cases,  the  guys  should 
extend  from  a  point  near  the  top 
of  the  pole  to  a  point  near  the 
butt  of  the  adjacent  pole. 
Straight  guying  should  also  be 
employed  at  the  terminal  pole, 
the  guy  extending  to  a  stub 
beyond  the  last  pole,  to  counter- 
act the  strain  of  the  wires  pull- 
ing in  the  opposite  direction.  On 
particularly  heavy  lines,  it  is 
sometimes  necessary  to  use 
straight  guys  for  the  second  and 
even  the  third  pole  from  the  ter- 
minal pole,  to  prevent  undue 
strain  on  the  terminal  pole  itself, 
as  shown  in  Fig.  65. 

Where  there  are  three  or 
more  cross-arms,  either  two  sets 
of  guys  should  be  employed,  or 
else  a  "Y"  form  of  guy  should 
be  used.  If  a  single  guy  is  used 
on  a  long  pole  or  on  a  pole  car- 
rying a  number  of  cross-arms,  or 
on  which  there  is  unusual  strain, 
the  pole  is  apt  to  break  where 
the  guy  is  attached.  Figs.  66  and 
67  show  respectively  a  proper  and 
an  improper  method  of  guying, 
and  their  effect. 


74 


ELECTRIC  WIRING 


At  corners,  or  wherever  the  direction  of  the  linework  changes, 
guys  should  be  provided  to  counteract  the  strain  due  to  the  change  in 
direction.  Guys  are  also  necessary  at  points  where  poles  are  set  in 
other  than  a  vertical  position. 

Where  the  soil  is  not  firm  or  solid,  or  where  poles  are  subject  to 
unusual  stress,  it  is  sometimes  necessary  to  obtain  additional  stiffness 
by  what  is  known  as  crib-bracing,  as  may  be  seen  from  Fig.  68.  This 
consists  of  placing  two  short  logs  at  the  butt  of  the  pole.  These 
logs  need  not  be  more  than  4  to  5  feet  long,  or  more  than  8  to  9  inches 


Fig.  66.    Proper  Method  of  Guying  where  there  are  Three  or  More  Cross- Arms. 
A  Y-form  of  Guy  is  Used  at  Left;  Double  Guy  at  Right. 

in  diameter.  This  crib-bracing  is  sometimes  also  necessary  to  give 
greater  stability  to  stubs  or  short  poles  to  which  guys  are  fastened. 

Wrhile,  as  a  rule,  it  is  not  advisable  to  use  trees  for  guy  supports, 
it  is  sometimes  necessary  to  do  this,  but  the  trees  should  be  sound  and 
should  be  protected  in  a  proper  manner  from  injury.  On  private 
property,  permission  should  first  be  obtained  from  the  owner  to  use 
the  tree  for  such  purpose. 

The  guy  itself  should  be  of  standard  cable,  consisting  of  7  strands 
of  No.  12  B.  &  S.  Gauge  iron  or  steel  wire.  This  is  the  standard 
guy  cable,  and  should  be  used  in  all  cases,  except  for  very  light  poles 
and  light  linework,  where  a  smaller  cable  having  a  minimum  diameter 
of  \  inch  may  be  used.  The  guy  wires  should  be  fastened  at  the  ends 
by  means  of  suitable  clamps.  All  guy  cables  and  clamps  should  be 
heavily  galvanized,  to  prevent  rusting, 


ELECTRIC  WIRING 


75 


Corners.     In  cases  of  heavy  linework  where  there  are  a  con- 
siderable number  of  wires  and  cross-arms,  the  turns  should  be  made, 


Fig.  67.    Improper  Method  of  Guying  where  there  are  Three  or  More  Cross-Arms. 
Strain  is  Concentrated  at  one  Point,  Causing  Rupture  of  Pole. 

if  possible,  by  the  use  of  two  poles.  In  cases  where  there  are  only  a 
few  wires,  a  double  cross-arm  may  be  employed,  using  a  single  pole. 
The  two  methods  are  illustrated  in  Figs.  69  and  70. 


Fig.  68.    Additional  Stiffness  Secured  by  Use  of  Crib-Bracing. 

Cross-Arms.    Cross-arms,  where   possible,   should   be  of  long 
leaf  yellow  pine,  or  of  Oregon  or  Washington  fir,  of  sound  wood, 


76 


ELECTRIC  WIRING 


thoroughly  seasoned  and  free  from  sap,  cracks,  cr  large  knots.  They 
should  be  not  less  than  3J  inches  thick  by  4J  inches  deep,  the  length 
depending  upon  the  number  of  pins  required. 

Cross-arms,  after  being  properly  seasoned,  should  be  painted 
with  two  coats  of  lead  paint  before  erection.  They  should  then  be 
snugly  fitted  into  the  gain  of  the  pole,  and  securely  fastened  with 
a  bolt  not  less  than  f  inch  in 
diameter  driven  through  a 
hole  of  slightly  less  diameter 
previously  bored  in  the  pole. 
A  galvanized-iron  washer 
not  less  than  2  inches 
diameter  should  be  placed 
under  the  head  and  nut  of 


Fig.  69.    Two-Poles  Used  in  Making  Turn 
on  Heavy  Line. 


each  bolt.  The  cross- 
arms  should  be  at  right 
angles  to  the  pole,  and 
should  be  parallel  to  one 
another  where  two  or 
more  arms  are  used  on 
the  same  pole. 

The  cross-arms 
should   be    braced    with 

galvanized-iron  braces  approximately  1J  inches  wide,  \  inch  thick, 
and  from  18  to  30  inches  in  length.  The  braces  should  be  fastened 
to  the  cross-arm  by  means  of  f-inch  galvanized-iron  bolts  passing 
through  the  brace  and  the  cross-arm,  washers  being  used  under  the 
nut  and  head  of  each  bolt.  Guys  should  be  provided  for  the  cross- 
arms  in  case  of  unusual  strain.  The  dimensions  of  cross-arms  re- 
quired for  various  numbers  of  pins,  are  given  verv  completely  in  a 


ELECTRIC  WIRING 


77 


O 


LJ 


paper  read  by  Mr.  Paul  Spencer  before  the  Atlantic  City  Convention 
of  the  National  Electric  Light  Association  in  1906,  and  reprinted 
in  a  number  of  the  technical  journals. 

Wherever  practicable,  cross-arms  should  be  placed  on  the  poles 
before  the  poles  are  erected,  as  not  only  can  they  be  more  securely 
fastened  when  the  poles  are  on  the  ground,  but  the  cost  of  erection 
is  thereby  considerably  reduced. 

Pins.  Pins  should  be  of  selected  locust,  not  less  than  f  inch 
diameter  at  the  shank  portion,  and  not  less  than  1 J  inches  in  diameter 
at  the  point  where 
they  rest  upon  the 
cross-arm.  For  po- 
tentials of  20,000 
volts  or  over,  the 
pins  should  be  of  ( 
metal,  to  avoid  car- 
bonization of  the  ( 
wood  due  to  static 
leakage.  The  top 
portion  of  the  pin 
(if  of  wood)  should 
be  not  less  than  one 
inch  in  diameter. 
The  length  of  both 
the  shank  and  the 
upper  portion  Fig>70> 
should  be  each  ap- 
proximately 4J  inches,  making  the  total  length  approximately  9 
inches.  The  pin  should  be  threaded  and  tapered,  and  accurately  cut. 
The  pin  should  fit  the  hole  in  the  cross-arm  snugly,  and  should  be 
nailed  to  the  cross-arm  with  a  sixpenny  galvanized-iron  wire  nail 
driven  straight  through  the  center  of  the  shank  of  the  pin. 

Insulators.  For  potentials  of  3,000  volts  or  less,  insulators 
should  be  of  flint  glass,  of  double-petticoat,  deep-grooved  type.  For 
potentials  of  over  3,000  volts,  they  should  be  of  the  triple-petticoat 
type,  and  preferably  of  porcelain,  and  should  be  of  special  pattern 
adapted  for  the  potential. 

Service    Mains,  Pole  Wiring,  etc.    For  service  connections- 


Double  Cross-Arm  Used  on  Single  Pole  to  Make  Turn 
in  Heavy  Line  Carrying  Only  a  Few  Wires. 


78  ELECTRIC  WIRING 

that  is,  for  the  mains  run  to  service  switches  in  consumers'  residences 
or  other  buildings,  conductors  of  not  less  than  No.  8  B.  &  S.  Gauge 
should  be  used  in  order  to  obtain  the  necessary  tensile  strength. 
Where  possible,  the  circuits  should  be  arranged  in  such  a  manner  as  to 
have  the  service  main  connect  with  the  line  on  the  lowest  cross-arm, 
in  order  to  prevent  crossing  of  wires.  The  transformers  should  be 
installed  either  on  poles  or  in  vaults  outside  of  the  building,  or,  where 
this  is  impracticable,  in  a  fireproof  vault  or  other  enclosed  space 
inside  of  the  building  itself.  Small  transformers  may  be  fastened  to 
a  pair  of  cross-arms  secured  to  the  pole  itself.  For  transformers  of 
25  K.  W.  and  over,  it  is  usually  best  to  provide  special  poles.  It  is 
inadvisable  to  place  transformers  on  building  walls. 

Where  appearance  is  of  importance,  when  the  transformer  is 
placed  underground,  or  when  the  wires  enter  the  lower  portion  of  a 
building,  the  conductors  must  be  run  underground.  In  such  cases,  a 
splice  should  be  made  between  the  weatherproof  conductors  and 
rubber-insulated  lead-sheathed  conductors,  at  a  height  of  about  15 
to  20  feet  above  the  ground,  and  the  mains  run  in  iron  pipe  down  the 
pole  to  a  point  underground,  where  they  may  be  continued  either  in 
iron  pipe  or  in  vitrified  or  fiber  conduits  underground  to  the  point 
of  entrance. 

All  circuit  wiring  on  poles  should  be  so  arranged  as  to  leave  one 
side  free  for  the  linemen  to  climb  the  poles  without  injuring  the  con- 
ductors. As  a  rule,  all  poles  on  which  transformers,  lightning  arresters, 
or  fuse-boxes  are  located,  should  be  provided  with  steps. 

In  order  to  limit  the  area  of  disturbance  of  a  short  circuit  or 
overload,  fuses  should  be  inserted  in  each  leg  of  a  primary  circuit 
in  making  connections  to  transformers,  or  where  tap  or  branch  con- 
nections are  made.  The  fuses  should  have  a  capacity  of  approxi- 
mately 50  per  cent  greater  than  the  transformer  or  conductor  which 
they  protect.  Of  course,  it  would  be  undesirable  to  have  an  excessive 
number  of  fuses,  and  for  short  branch  lines  they  might  frequently 
be  undesirable;  but  for  important  branch  lines,  they  should  be  em- 
ployed in  order  to  prevent  the  fuse  on  the  main  feeder  from  being 
blown  in  case  of  disturbance  en  the  branch  line. 

Lightning  arresters  should  be  placed  on  the  linework  in  places 
particularly  exposed  to  lightning  discharges,  and  at  all  points  where 
connections  are  made  to  enter  a  building.  The  location  and  number 


ELECTRIC  WIRING 


79 


of  lightning  arresters  will  depend  upon  local  conditions,  the  likelihood 
and  frequency  of  thunderstorms,  etc.  Where  lightning  arresters  are 
provided,  it  is  essential  that  a 
good  ground  connection  be  obtained. 
The  ground  connection  should  be 
made  by  a  fairly  good-sized  insu- 
lated rubber  conductor,  not  less 
than  No.  6  B.  &  S.  Gauge,  con- 
necting either  with  a  water  pipe 
to  which  it  should  be  clamped,  or 
fastened  in  such  a  manner  as  to 
obtain  a  good  electric  contact,  or 
else  to  a  ground-plate  of  copper 
embedded  in  crushed  charcoal  or 
coke. 

The  neutral  wire  of  a  three- 
wire  of  both  secondary  alternating- 
current  systems  and  direct-current 
systems,  should  be  properly 


Fig.  71.    Method  of  Wiring  to  and  Sup- 
porting Lamp  on  Pole. 


grounded  as  required  by  the  National  Electric  Code  (see  Rules  12, 
13,  and  13-A). 

Lamps  on  Poles.    Fig.  71  shows  the  method  of  wiring  to  and 
supporting  a  lamp  located  on  a  pole. 


UNDERGROUND    LINEWORK 

In  large  cities,  or  in  congested  districts,  or  where  the  appearance 
of  overhead  linework  is  objectionable,  it  is  generally  necessary  to 
place  the  conductors  underground  instead  of  overhead. 

The  advantages  of  underground  linework  are — first,  that  of 
appearance;  second,  it  is  more  permanent  and  less  liable  to  inter- 
ruption than  overhead  work. 

The  principal  disadvantage  of  underground  work  is  the  greater 
first  cost.  In  overhead  linework,  conductors  having  weatherproof 
insulators  consisting  of  cotton  dipped  in  a  special  compound  similar 
to  pitch,  are  used,  the  cost  of  which  is  relatively  small.  For  under- 
ground linework,  however,  the  conductors  must  not  only  have  rubber 
insulation,  but  also  a  lead  sheathing  for  mechanical  protection. 


80  ELECTRIC  WIRING 

Furthermore,  the  cost  of  the  ducts,  trenching,  concrete  work,  laying 
the  ducts,  etc.,  is  much  greater  than  the  cost  of  poles,  cross-arms,  etc. 

As  in  the  case  of  inside  wiring,  underground  linework  should 
be  so  arranged  that  the  conductors  may  be  readily  removed  and  re- 
placed without  disturbing  the  underground  conduits  or  ducts.  The 
system  should  be  arranged  with  manholes,  and  in  such  a  manner  that 
changes  or  additions  or  branches  may  be  readily  and  conveniently 
made.  In  order  to  provide  for  the  removal  and  replacing  of  con- 
ductors, and  also  for  growth  in  the  system,  the  method  formerly  in 
vogue,  of  embedding  the  conductors  in  wooden  boxes,  or  in  trenches 
underground,  has  been  abandoned;  and  the  conductors  are  now 
placed  in  conduits  or  ducts.  A  number  of  different  forms  of  ducts  and 
conduits  have  been  introduced,  some  of  which  have  been  dropped  as 
cheaper  and  better  forms  have  been  introduced.  The  forms  of  con- 
duits or  ducts  now  most  generally  employed  include  iron  pipe,  vitrified 
conduits,  and  fibre  conduit.  As  all  three  of  these  forms  of  conduit 
are  very  generally  employed,  they  will  now  be  described,  as  well  as  the 
method  of  installing  them. 

Iron  Pipe.  Three-inch  iron  conduit  is  frequently  used  for  under- 
ground linework,  particularly  for  short  runs  or  where  there  are  not 
more  than  two  or  three  ducts  required,  or  where  the  soil  is  bad  and 
where  the  longer  lengths  and  more  stable  joints  of  the  iron  conduit 
would  make  it  more  desirable  than  vitrified  duct  or  fibre  conduit. 
This  conduit,  however,  is  generally  undesirable  on  account  of  its 
greater  first  cost,  and  also  on  account  of  its  liability  to  deterioration 
from  rust  or  corrosion.  Where  iron  conduit  is  used,  and  where  it  is 
subject  to  corrosion,  it  should  be  coated  with  asphaltum  or  other 
similar  protective  composition.  While  it  is  not  necessary  to  have  a 
concrete  bed  under  iron  pipe,  it  is  better  to  provide  such  a  bed,  especi- 
ally where  the  soil  is  shifting  or  not  solid. 

Vitrified  Tile  Conduit.  This  type  of  conduit  in  both  the  single- 
and  multiple-duct  form,  is  used  more  extensively  than  any  other  form 
of  conduit  for  underground  work.  It  is  made  in  lengths  of  18  inches 
for  the  single-duct  form,  and  in  considerably  greater  lengths  in  the 
multiple-duct  form.  Fig.  72  shows  the  single-duct  conduit,  and 
Fig.  73  shows  a  multiple-duct  form  of  conduit. 

Vitrified  conduit  requires  less  space  for  the  same  number  of 
ducts  than  any  other  form,  and  is  particularly  desirable  where  a  great 


ELECTRIC  WIRING 


81 


number  of  ducts  are  required  in  a  small  space.  The  advantages  of 
this  form  of  conduit  are  that  it  is  cheap  in  first  cost;  after  being 
laid,  it  is  practically  indestructible;  it  is  not  subject  to  corrosion  or 


Fig.  72.    Self-Centering  Duct, 

Vitrified  Conduit. 

Courtesy  of  Standard  Vitrified  Conduit  Co., 
New  York,  N.  Y. 


Fig.  73.    Multiple  Duct,  Vitrified 

Conduit. 

Courtesy  of  Standard  Vitrified  Conduit  Co., 
New  York,  N.  Y. 


deterioration;  it  is  not  combustible;  it  is  fairly  strong  mechanically; 
and  it  does  not  require  skilled  labor  to  install. 

Table  XII  gives  the  principal  data  of  one  of  the  well-known 
makes  of  vitrified  conduit  • 

TABLE  XII 
Standard  Vitrified  Conduit 


STYLE  OF  CONDUIT 

DIMENSION 
OF  SQUARE 
DUCT 
(INCHES) 

DIMENSION 
OF  ROUND 
DUCT 
(INCHES) 

OUTSIDE 
DIMENSIONS 
OF  END  SEC- 
TION (!N.) 

REG. 
STOCK 
LENGTHS 

(INCHES) 

SHORT 
LENGTHS 
(INCHES) 

APPROX. 
WEIGHT 

PERDUCT 

(FOOT) 

2-ducfc  multiple.  .  . 
3-duct  multiple.  .  . 
4-duct  multiple.  .  . 
6-duct  multiple.  .  . 
9-duct  multiple.  .  . 
Common  single 
duct  

3|  sq. 
3|  sq. 
3f  sq. 
3f  sq. 
3f  sq. 

COCOCOCOCO  CO  CO  CO 

f 
\ 

5x    9 
5x13 
9x    9 
9x  13 
13x  13 

5x5 
5x5 
5  in.  round 

24 
24 
36 
38 
36 

18 
18 
18 

6,  9,  12 
6,9,  12 
6,  9,  12 
6,9,  12 
6,9,  12 

6,9,  12 
6,9,  12 
6,9,  12 

0000000000  00  O  O 
1—  I  1—  I 

Single  duct,  self- 
centering  

Round  single  duct, 
self-centering.  .  . 

In  installing  vitrified  conduit,  a  trench  following  as  straight 
lines  as  possible  should  be  dug  to  such  a  depth  that  there  will  be  a 
space  of  at  least  18  inches  from  the  top  layer  of  the  duct  to  the  street 
surface.  The  bottom  of  the  trench  should  be  level;  and  a  bed  of 
good  cement  concrete  not  less  than  3  inches  thick  should  be  laid. 
The  following  instructions*  for  installing  vitrified  conduit  may  be 
considered  as  typical  of  the  best  up-to-date  practice: 

*From  the  Catalogue  of  the  Standard  Underground  Conduit  Company. 


82  ELECTRIC  WIRING 

Laying  of  Conduit.  When  the  trench  has  been  properly  pre- 
pared and  the  concrete  foundation  set,  the  laying  of  conduit  should  be 
begun.  The  ends  of  the  conduit  should  be  butted  against  the  shoulder 
of  the  conduit  terminal  brick;  short  length  should  be  used  for  the 
breaking  of  joints. 

Care  should  be  taken,  when  each  length  of  conduit  is  laid,  that 
the  duct  hole  is  perfectly  clear  and  the  conduit  level.  The  work  may 
then  proceed;  and  if  the  following  instructions  are  carried  out,  no 
difficulty  will  be  encountered  after  the  duct  are  laid. 

When  the  first  piece  of  conduit  is  laid  and  the  keys  inserted, 
one  on  the  top  and  one  on  the  side  of  the  duct,  the  burlap  for  joints 
should  be  slipped  partly  under  the  conduit,  and  the  next  piece  brought 
up  and  connected.  The  burlap  is  then  brought  up  and  wrapped 
around  the  conduit.  After  this  operation  is  completed,  a  thin  layer 
of  cement  mortar  is  plastered  around  the  burlap,  extending  over  the 
edges,  so  as  to  cover  the  scarified  portion  of  the  conduit  so  that  it 
may  adhere  to  it,  thus  making  the  joint  practically  water-tight. 

The  burlap  should  be  first  prepared  in  strips  of  not  less  than  6 
inches  in  width,  and  of  suitable  length  to  wrap  around  the  conduit, 
overlapping  about  6  inches.  If  possible,  the  burlap  should  be  satur- 
ated in  asphaltum  or  pitch;  but  if  this  is  not  convenient,  it  may  be 
dipped  in  water  so  as  to  stick  to  the  conduit  until  the  joint  has  been 
cemented.  The  engineer  or  foreman  in  charge  should  personally 
oversee  the  making  of  the  joint,  and  especially  see  that  the  keys  are 
inserted,  as  in  many  instances  they  are  left  out  by  the  workmen, 
causing  considerable  trouble  and  expense.  Sufficient  time  should  be 
allowed  for  the  joints  to  harden. 

After  the  duct  are  laid,  the  sides  are  filled  in  with  either  concrete 
or  dirt,  as  specified,  care  being  taken  that  the  conduit  are  not  forced 
out  of  alignment  by  the  careless  filling-in  of  the  sides.  The  top  layer 
of  concrete  may  then  be  laid  and  leveled. 

After  this  the  trench  is  ready  for  filling  in. 

In  the  laying  of  our  self-centering  single-duct  conduit,  no  dowel- 
pins  are  used,  the  ducts  being  self-centering — one  piece  of  conduit 
socketing  into  the  other.  Burlaping  and  cementing  of  joint  is  not 
necessary.  Otherwise  the  instructions  for  the  laying  of  multiple- 
duct  should  be  followed.  The  use  of  a  mandrel  in  laying  self- 
centering  conduit  is  superfluous. 


ELECTRIC  WIRING  83 

As  each  section  of  the  system — that  is,  from  manhole  to  man- 
hole— is  completed,  it  should  be  rodded  to  insure  the  duct  being  clear. 
For  this  purpose  wooden  rods  are  employed,  the  rods  being  from 
3  to  4  feet  long  by  one  inch  in  diameter  and  provided  with  brass 
couplings  on  the  ends.  The  first  rod  is  pushed  into  the  duct  chamber, 
the  second  one  is  then  attached,  and  then  the  third  and  so  on,  until  the 
first  rod  appears  in  the  manhole  at  the  opposite  end, 

A  wooden  mandrel  about  10  inches  long,  made  to  conform  to 
the  shape  of  the  duct,  but  about  J  inch  smaller  in  diameter,  is  attached 
to  the  last  rod,  and  a  galvanized -iron  wire  is  then  attached  to  the  other 
end  of  the  mandrel.  The  rods  are  drawn  through  the  duct  and 
uncoupled,  until  the  mandrel  has  passed  through  the  ducts.  The 
wire  is  left  remaining  in  the  chamber,  and  secured  in  the  manhole  to 
prevent  its  being  pulled  out.  The  same  operation  is  repeated  until 
all  the  ducts  are  tested  and  wired.  Should  obstructions  be  met  with 
and  the  mandrel  bind,  the  location  of  the  obstructions  can  readily  be 
ascertained  from  the  length  of  rod  yet  remaining  in  the  duct,  and  can 
easily  be  removed.  This  method  is  far  better  than  pulling  the 
mandrel  through  as  the  ducts  are  laid,  as  in  many  cases  the  duct  is 
obstructed  or  thrown  out  of  alignment  by  the  filling-in  of  the  con- 
crete or  trench,  and  this  would  not  be  noticed  until  an  attempt  was 
made  to  draw  the  cable.  The  wire  left  in  the  duct  is  used  in  drawing 
the  cables. 

Fibre  Conduit.  This  type  of  conduit  consists  of  wood  fibre 
formed  into  a  tube  over  a  mandrel  under  pressure.  After  the  tube 


Fig.  74.    Socket-Joint  Fibre  Conduit. 

is  formed  on  the  mandrel,  it  is  removed,  and,  after  being  dried  in 
air,  is  placed  in  a  tank  of  preservative  and  insulating  compound. 

Fibre  conduits  are  made  in  three  different  styles — namely,  the 
socket- joint,  sleeve- joint,  and  screw- joint  types,  shown  respectively  in 
Figs.  74,  75,  and  76.  The  forms  of  conduit  here  shown  are  made  by 
the  Fibre  Conduit  Company,  of  Orangeburg,  New  York. 

In  the  socket-joint  type,  the  connections  between  the   lengths 


84 


ELECTRIC  WIRING 


of  conduit  are  made  by  means  of  male  and  female  joints  turned  on 
the  ends  of  the  conduit  so  that  it  is  necessary  only  to  push  one  length 
within  the  other  to  secure  alignment  without  the  use  of  a  sleeve- 
coupling  or  other  device.  While  this  is  the  cheapest  and  simplest 


Fig.  75.    Sleeve-Joint  Fibre  Conduit. 

form  of  fibre  conduit,  the  joint  is  not  so  secure  as  in  either  of  the  other 
two  types. 

The  sleeve-joint  fibre  conduit  has  the  ends  of  each  joint  turned 
so  that  a  sleeve  may  be  slipped  over  the  turned  portion  and  butted  up 
against  the  shoulder  on  the  tubes.  These  sleeves  are  about  4  inches 
long  and  f  inch  thick.  While  this  form  of  joint  is  more  secure  than 
the  socket  type,  it  is  not  so  secure  as  the  screw-joint  type. 

The  screw-joint  type  of  fibre  conduit  is  manufactured  with  a 
slightly  thicker  wall  than  the  socket-joint  type,  in  order  to  obtain  the 
necessary  thickness  for  getting  the  thread  on  the  end  of  the  pipe.  The 
sleeve  in  this  case  is  threaded;  and,  instead  of  being  slipped  on  the 
conduit,  as  in  the  case  of  the  sleeve-joint  type,  it  is  screwed  on,  and 
the  thread  may  be  filled  with  compound  and  a  water-tight  joint  thereby 
obtained.  Various  special  forms  of  elbows,  bends,  junction-boxes, 
tees,  etc.,  are  provided  for  this  conduit,  for  special  connections. 
Couplings  are  also  made  so  that  joints  can  be  made  between  fibre 
conduit  and  iron  pipe,  where  it  is  desirable  to  make  such  a  connection. 

The  advantages  of  fibre  conduit  are — -first,  that  it  is  lighter  than 
any  of  the  other  forms  of  conduit,  which  reduces  the  cost  of  trans- 


Fig.  76.    Screw-Joint  Fibre  Conduit. 


portation,  carting,  and  handling;  and  second,  that  the  cost  of  labor 
for  installing  it  is  less  than  in  the  case  of  iron  pipe,  and  less  than  that 
of  the  single-duct  tile  pipe.  Table  XIII  edves  the  principal  data 
relating  to  fibre  conduit, 


ELECTRIC  WIRING 


85 


TABLE  XIII 

Fibre  Conduit 


INSIDE 
DIAMETER 
(INCHES) 

TYPE  OF   CONDUIT 

LENGTH 
(FEET) 

THICKNESS  op 
WALL,  (INCHES) 

APPROX. 

WEIGHT  PER 
FOOT  (LBS.) 

1 

Socket  -joint 

2-i 

0.50 

1* 

5 

0.70 

2 

5 

J 

0.85 

2i 

5 

1.02 

3 

5 

1.20 

3£ 

5 

1.40 

4 

5 

1.60 

li 

Sle 

eve-joi 

it 

5 

0.80 

2 

5 

0.95 

2| 

5 

i 

1.15 

3 

5 

176 

2.40 

3* 

5 

A 

2.90 

4 

5 

J 

3.33 

1* 

Scr 

ew-joir 

t 

5 

I5e 

1.00 

2 

5 

1 

1.45 

2* 

5 

f 

1.75 

3 

5 

T76 

2.40 

3£ 

5 

TV 

2.90 

4 

5 

J 

3.33 

Fig.  77  shows  the  method  of  laying  fibre  conduit  in  a  trench. 

A  concrete  bed  should  be  provided  for  all  three  types  of  fibre 
conduit.  Where  the  ground  is  moist  or  where  there  is  likelihood  of 
water  getting  in  the  joints,  it  is  advisable  to  make  a  complete  envelope 
around  the  conduit. 

The  joints  should  be  carefully  dipped  in  or  coated  with  a  special 
liquid  compound  provided  for  this  purpose,  so  as  to  insure  water- 
tightness.  The  cables  should  be  spaced  about  1J  inches  apart,  by 
means  of  wooden  separators;  and  the  spaces  between  the  ducts,  and 
between  the  walls  of  the  trench  and  the  outer  ducts,  should  be  filled 
with  a  thin  grouting  of  cement  and  sand.  If  more  than  one  horizontal 
row  of  ducts  are  installed,  the  grouting  of  each  row  should  be  smoothed 
over  so  as  to  prepare  a  base  for  the  next  row  of  ducts. 

To  fish  the  conductors  in  fibre  conduit,  it  is  not  necessary  to  fol- 
low the  method  of  rodding  usually  required  with  vitrified  conduits; 
but  it  is  found  that  by  utilizing  a  solid  No.  6  iron  wire,  and  fishing 
from  one  manhole  to  the  next,  the  mandrels  and  brush  can  be  attached 
to  the  end  of  the  wire  and  pulled  through  the  conduits,  thus  insuring 
that  the  joints  are  smooth  and  that  there  are  no  obstructions  in  the 
conduit.  To  prevent  accidental  clogging  of  the  ends  of  the  con- 


86 


ELECTRIC  WIRING 


duit,  wooden  plugs  should  be  installed  in  the  openings  of  all  un- 
finished conduit  work,  or  in  all  unoccupied  cable  ducts  at  manholes. 
Drawing  In  the  Cables.  After  the  conduits  have  been  tested  by 
means  of  the  mandrel  to  ascertain  that  they  are  continuous  and  that 
the  joints  are  smooth,  the  work  of  installing  the  cables  may  be  started. 
Special  precaution  should  be  taken  to  prevent  sharp  bending  of  the 
cables,  and  thus  to  prevent  injury  to  the  lead  sheathing  of  the  rubber 

insulation.  If  the 
cable  is  light  and 
of  small  diam- 
eter, the  distance 
not  over  300  feet, 
and  the  run  fairly 
straight,  the  ca- 
ble can  usually 
be  pulled  in  by 
hand;  but  often 
other  mean  s 
must  be  provided 
so  as  to  secure 
sufficient  power. 
Pr  e  cautions 
should  be  taken, 
however,  to 
avoid  placing  too 
great  a  strain  on 
the  cables,  as  it 
is  liable  to  in- 
jure them,  and 
the  injuries  may 

not  show  up  immediately,  but  may  cause  trouble  later.  The  remedy 
is  to  avoid  placing  the  manholes  too  far  apart,  and  to  have  the  runs  as 
straight  as  possible;  also  to  properly  test  the  conduits  for  continuity 
and  smoothness  before  starting  to  install  the  cables.  Enough  slack 
should  be  left  in  each  manhole  to  allow  the  cables  to  pass  close  to 
the  side  walls  of  the  manhole,  and  to  have  the  centers  free  and  acces- 
sible for  a  man  to  enter  the  manhole.  Where  there  are  a  great 
number  of  cables  in  a  manhole,  shelves  or  other  supports  should  be 


Fig.  77.    Method  of  Laying  Fibre  Conduit  in  Trench. 


ELECTRIC  WIRING 


87 


provided  for  holding  the  cables  apart  and  in  position.  Where  two 
or  more  conductors  are  placed  in  the  same  duct,  they  should  always 
be  pulled  in  at  the  same  time,  for  otherwise  the  cables  last  pulled  in 
are  apt  to  injure  those  already  installed. 

Manholes.  Manholes  should  be  provided  about  every  300  feet, 
in  order  to  facilitate 
the  installation  of  the 
conductors  in  the  duct. 
The  exact  distance  be- 
tween manholes 
should  be  determined 
by  conditions;  in  some 
?ases  they  should  be 
placed  even  closer  to- 
gether than  the  figure 
given,  while  in  other 
cases  their  distance 
apart  mi  gh  t  be 
slightly  greater. 

Manholes  are 
built  of  concrete  or 
brick,  and  provided 
with  a  cast-iron  frame 
or  cover.  The  man- 
holes  may  be  of 
square,  round,  rect- 
angular, or  oval  sec- 
tion, the  last-men-  Fig. 78. 
tioned  form  of  man- 
hole being  probably  the  best,  as  it  avoids  the  liability  to  sharp  bends 
or  kinks  being  made  in  the  cable.  The  manhole  cover  may  be  of 
the  same  form  as  the  manhole  itself,  or  it  may  be  of  different  form;  but 
round  or  square  covers  are  usually  used.  Fig.  78  shows  a  standard 
form  of  manhole  used  in  New  York  City.  This  manhole  is  substan- 
tially built,  and  adapted  for  heavy  traffic  passing  over  the  cover.  For 
suburban  or  country  work,  manholes  may  be  made  of  lighter  con- 
struction. 


Plan  and  Sectional  Elevation  of  Standard  Form 
of  Manhole  Used  in  New  York  City. 


AERIAL  CONSTRUCTION 
Telephone  and  Electric  Light  Wires. 


ELECTRIC  LIGHTING 


HISTORY  AND  DEVELOPMENT 

The  history  of  electric  lighting  as  a  commercial  proposition  begins 
with  the  invention  of  the  Gramme  dynamo,  by  Z.  J.  Gramme,  in 
1870,  together  with  the  introduction  of  the  Jablochkoff  candle  or 
light,  which  was  first  announced  to  the  public  in  1876,  and  which 
formed  a  feature  of  the  International  Exposition  at  Paris  in  1878. 
Up  to  this  time,  the  electric  light  was  known  to  but  few  investigators, 
one  of  the  earliest  being  Sir  Humphrey  Davy  who,  in  1810,  produced 
the  first  arc  of  any  great  magnitude.  It  was  then  called  the  voltaic 
arc,  and  resulted  from  the  use  of  two  wood  charcoal  pencils  as  elec- 
trodes and  a  powerful  battery  of  voltaic  cells  as  a  source  of  current. 

From  1840  to  1859,  many  patents  were  taken  out  on  arc  lamps, 
most  of  them  operated  by  clockwork,  but  these  were  not  successful, 
due  chiefly  to  the  lack  of  a  suitable  source  of  current,  since  all  de- 
pended on  primary  cells  for  their  power.  The  interest  in  this  form 
of  light  died  down  about  1859,  and  nothing  further  was  attempted 
until  the  advent  of  the  Gramme  dynamo. 

The  incandescent  lamp  was  but  a  piece  of  laboratory  apparatus 
up  to  1878,  at  which  time  Edison  produced  a  lamp  using  a  platinum 
spiral  in  a  vacuum,  as  a  source  of  light,  the  platinum  being  rendered 
incandescent  by  the  passage  of  an  electric  current  through  it.  The 
first  successful  carbon  filament  was  made  in  1879,  this  filament  being 
formed  from  strips  of  bamboo.  The  names  of  Edison  and  Swan  are 
intimately  connected  with  these  early  experiments. 

From  this  time  on,  the  development  of  electric  lighting  has  been 
very  rapid,  and  the  consumption  of  incandescent  lamps  alone  has 
reached  several  millions  each  year.  When  we  compare  the  small 
amount  of  lighting  done  by  means  of  electricity  twenty-five  years  ago 
with  the  enormous  extent  of  lighting  systems  and  the  numerous 
applications  of  electric  illumination  as  they  are  to-day,  the  growth 
and  development  of  the  art  is  seen  to  be  very  great,  and  the  value  of 
a  study  of  this  subject  may  be  readily  appreciated.  While  in  many 

Copyright,  1909,  by  American  School  of  Correspondence. 


2  ELECTRIC  LIGHTING 

cases  electricity  is  not  the  cheapest  source  of  power  for  illumination, 
its  admirable  qualities  and  convenience  of  operation  make  it  by  far 
the  most  desirable. 

CLASSIFICATION 

The  subject  of  electric  lighting  may  be  classified  as  follows: 

1.  The  type  of  lamps  used. 

2.  The  methods  of  distributing  power  to  the  lamps. 

3.  The  use  made  of  the  light,  or  its  application. 

4.  Photometry  and  lamp  testing. 

The  types  of  lamps  used  may  be  subdivided  into: 

1.  Incandescent  lamps:  Carbon,  metallic  filament;  Nernst. 

2.  Special  lamps:  Exhausted  bulb  without  filament,  such  as  the  Cooper- 
Hewitt  lamp  and  Moore  tube  lamp. 

3.  Arc  lamps:  Ordinary  carbon,  flaming  arc. 

INCANDESCENT  LAMPS 

The  incandescent  lamp  is  by  far  the  most  common  type  of  lamp 
used,  and  the  principle  of  its  operation  is  as  follows: 

If  a  current  I  is  sent  through  a  conductor  whose  resistance  is 
R,  for  a  time  t,  the  conductor  is  heated,  and  the  heat  generated  = 
PR  t,  PR  t  representing  joules  or  watt-seconds. 

If  the  current,  material,  and  conditions  are  so  chosen  that  the 
substance  may  be  heated  in  this  way  until  it  gives  out  light,  becomes 
incandescent,  and  does  not  deteriorate  too  rapidly,  we  have  an  in- 
candescent lamp.  Carbon  was  the  first  successful  material  to  be 
chosen  for  this  conductor  and  for  ordinary  lamps  it  is  formed  into  a 
small  thread  or  filament.  Very  recently  metallic  filament  lamps 
have  been  introduced  commercially  with  great  success  but  the  carbon 
incandescent  lamp  will  continue  to  be  used  for  some  time,  especially 
in  the  low  candle-power  units  operated  at  commercial  voltages.  Car- 
bon is  a  successful  material  for  two  reasons: 

.1.  The  material  must  be  capable  of  standing  a  very  high  tem- 
perature, 1,280°  to  1,330°  C.,  or  even  higher. 

2.  It  must  be  a  conductor  of  electricity  with  a  fairly  high  re- 
sistance. 

Platinum  was  used  in  an  early  stage  of  the  development,  but, 
as  we  shall  see,  its  temperature  cannot  be  maintained  at  a  value  high 
enough  to  make  the  lamp  as  efficient  as  when  carbon,  or  a  metal 


ELECTRIC  LIGHTING  3 

having  a  melting  point  higher  than  that  of  platinum,  is  used.  Nearly 
all  attempts  to  substitute  another  substance  in  place  of  carbon  have 
failed  until  recently,  and  the  few  lamps  which  are  entirely  or  partially 
successful  will  be  treated  later.  The  nature  of  the  carbon  employed 
in  incandescent  lamps  has,  however,  been  much  improved  over  the 
first  forms,  and  owing  to  the  still  very  great  importance  of  this  lamp, 
the  method  of  manufacture  will  be  considered. 

Manufacture  of  Carbon  Incandescent  Lamps.  Preparation  of 
the  Filament.  Cellulose,  a  chemical  compound  rich  in  carbon,  is 
prepared  by  treating  absorbent  cotton  with  zinc  chloride  in  proper 
proportions  to  form  a  uniform,  gelatine-like  mass.  It  is  customary 
to  stir  this  under  a  partial  vacuum  in  order  to  remove  bubbles  of  air 
which  might  be  contained  in  it  and  destroy  its  uniformity.  This 
material  is  then  forced,  "squirted,"  through  steel  dies  into  alcohol,  the 


W 

BODE. 

Fig.  1.     Fonr.s  of  Filaments  now  in  Use. 

alcohol  serving  to  harden  the  soft,  transparent  threads.  These  threads 
are  then  thoroughly  washed  to  remove  all  trace  of  the  zinc  chloride, 
dried,  cut  to  the  desired  lengths,  wound  on  forms,  and  carbonized  by 
heating  to  a  high  temperature  away  from  air.  During  carbonization, 
the  cellulose  is  transformed  into  pure  carbon,  the  volatile  matter  being 
driven  off  by  the  high  temperature  to  which  the  filaments  are  subjected. 
The  material  becomes  hard  and  stiff,  assuming  a  permanent  form, 
shrinking  in  both  length  and  diameter — the  form  being  specially  con- 
structed so  as  to  allow  for  this  shrinkage.  The  forms  are  made  of 
carbon  blocks  which  are  placed  in  plumbago  crucibles  and  packed 
with  powdered  carbon.  The  crucibles,  which  are  covered  with 
loosely  fitting  carbon  covers,  are  gradually  brought  to  a  white  heat, 
at  which  temperature  the  cellulose  is  changed  to  carbon,  and  then 
allowed  to  cool.  After  cooling,  the  filaments  are  removed,  measured, 
and  inspected,  and  the  few  defective  ones  discarded. 


4  ELECTRIC  LIGHTING 

In  the  early  days,  these  filaments  were  made  of  cardboard  or 
bamboo,  and  later,  of  thread  treated  with  sulphuric  acid. 

A  few  of  the  shapes  of  filaments  now  in  use  are  shown  in  Fig.  1, 
the  different  shapes  giving  a  slightly  different  distribution  of  light. 
As  here  shown  they  are  designated  as  follows:  A,  U-shaped;  B, 
single-curl;  C,  single-curl  anchored;  D,  double-loop;  E,  double- 
curl;  F,  double-curl  anchored. 

Mounting  the  Filament.  After  carbonization,  the  filaments 
are  mounted  or  joined  to  wires  leading  into  the  globe  or  bulb.  These 
wires  are  made  of  platinum — platinum  being  the  only  substance,  so 
far  as  known,  that  expands  and  contracts  the  same  as  glass,  with 
change  in  temperature  and  which,  at  the  same  time,  will  not  be  melted 
by  the  heat  developed  in  the  carbon.  Since  the  bulb  must  remain 
air-tight,  a  substance  expanding  at  a  different  rate  from  the  glass 
cannot  'be  used.  Several  methods  of  fastening  the  filament  to  the 
leading  in  wires  have  been  used,  such  as  forming  a  socket  in  the  end 
of  the  wire,  inserting  the  filament,  and  then  squeezing  the  socket 
tightly  against  the  carbon;  and  the  use  of  tiny  bolts  when  cardboard 
filaments  were  used;  but  the  pasted  joint  is  now  used  almost  exclu- 
sively. Finely  powdered  carbon  is  mixed  with  some  adhesive  com- 
pound, such  as  molasses,  and  this  mixture  is  used  as  a  paste  for  fasten- 
ing the  carbon  to  the  platinum.  Later,  when  current  is  sent  through 
the  joint,  the  volatile  matter  is  driven  off  and  only  the  carbon  remains. 
This  makes  a  cheap  and,  at  the  same  time,  a  very  efficient  joint. 

Flashing.  Filaments,  prepared  and  mounted  in  the  manner 
just  described,  are  fairly  uniform  in  resistance,  but  it  has  been  found 
that  their  quality  may  be  much  improved  and  their  resistance  very 
closely  regulated  by  depositing  a  layer  of  carbon  on  the  outside  of  the 
filament  by  the  process  of  flashing.  By  flashing  is  meant  heating  the 
filament  to  a  high  temperature  when  immersed  in  a  hydrocarbon  gas, 
such  as  gasoline  vapor,  under  partial  vacuum.  Current  is  passed 
through  the  filament  in  this  process  to  accomplish  the  heating.  Gas 
is  used,  rather  than  a  liquid,  to  prevent  too  heavy  a  deposit  of  the 
carbon.  Coal  gas  is  not  recommended  because  the  carbon,  when 
deposited  from  this,  has  a  dull  black  appearance.  The  effects  of 
flashing  are  as  follows: 

1.  The  diameter  of  the  filament  is  increased  by  the  deposited 
carbon  and  hence  its  resistance  is  decreased.  The  process  must  be 


ELECTRIC  LIGHTING 


discontinued  when  the  desired  resistance  is  reached.  Any  little  irregu- 
larities in  the  filament  will  be  eliminated  since  the  smaller  sections, 
having  the  greater  resistance,  will  become  hotter  than  the  remainder 
of  the  filament  and  the  carbon  is  deposited  more  rapidly  at  these 
points. 

2.  The  character  of  the  surface  is  changed  from  a  dull  black 
and  comparatively  soft  nature  to  a  bright  gray  coating  which  is  much 
harder  and  which  increases  the  life  and  efficiency  of  the  filament. 

Exhausting.  After  flashing,  the  filament  is  sealed  in  the  bulb 
and  the  air  exhausted  through  the  tube  A  in  Fig.  2,  which  shows  the 
lamp  in  different  stages  of  its 
manufacture.  The  exhaustion 
is  accomplished  by  means  of 
mechanical  air  pumps,  sup- 
plemented by  Sprengle  or  mer- 
cury pumps  and  chemicals. 
Since  the  degree  of  exhaustion 
must  be  high,  the  bulb  should 
be  heated  during  the  process 
so  as  to  drive  off  any  gas  which 
may  cling  to  the  glass.  When 
chemicals  are  used,  as  is  now 
almost  universally  the  case,  the 
chemical  is  placed  in  the  tube 
A  and,  when  heated,  serves 
to  take  up  much  of  the  remain- 
ing gas.  Exhaustion  is  neces- 
sary for  several  reasons: 

1.  To  avoid  oxidization  of  the  filament. 

2.  To  reduce  the  heat  conveyed  to  the  globe. 

3.  To  prevent  wear  on  the  filament  due  to  currents  or  eddies  in  the  gas. 

After  exhausting,  the  tube  A  is  sealed  off  and  the  lamp  com- 
pleted for  testing  by  attaching  the  base  by  means  of  plaster  of  Paris. 
Fig.  3  shows  some  of  the  forms  of  completed  incandescent  lamps. 

Voltage  and  Candle=Power.  Incandescent  lamps  of  the  carbon 
type  vary  in  size  from  the  miniature  battery  and  candelabra  lamps  to 
those  of  several  hundred  candle-power,  though  the  latter  are  very 
seldom  used.  The  more  common  values  for  the  candle-power  are 


Fig.  2.     Different  Stages  in  Lamp  Manufacture. 


ELECTRIC  LIGHTING 


8,  16,  25,  32,  and  50,  the  choice  of  candle-power  depending  on  the 
use  to  be  made  of  the  lamp. 

The  voltage  will  vary  depending  on  the  method  of  distribution 
of  the  power.  For  what  is  known  as  parallel  distribution,  110  or 
220  volts  are  generally  used.  For  the  higher  values  of  the  voltage, 
long  and  slender  filaments  must  be  used,  if  the  candle-power  is  to  be 
low;  and  lamps  of  less  than  16  candle-power  for  220- volt  circuits  are 
not  practical,  owing  to  difficulty  in  manufacture.  For  series  dis- 
tribution, a  low  voltage  and  higher  current  is  used,  hence  the  fila- 
ments may  be  quite  heavy.  Battery  lamps  operate  on  from  4  to  24 
volts,  but  the  vast  majority  of  lamps  for  general  illumination  are 
operated  at  or  about  110  volts. 


Fig.  3.     Several  Forms  of  Completed  Lamps. 

Efficiency.  By  the  efficiency  of  an  incandescent  lamp  is  meant 
the  power  required  at  the  lamp  terminals  per  candle-power  of  light 
given.  Thus,  if  a  lamp  giving  an  average  horizontal  candle-power 
of  16  consumes  J  an  ampere  at  112  volts,  the  total  number  of  watts 
consumed  will  be  112  X  J  =  56,  and  the  watts  per  candle-power 
will  be  56  -f-  16  =  3.5.  The  efficiency  of  such  a  lamp  is  said  to  be 
3.5  watts  per  candle-power,  or  simply  watts  per  candle.  Watts 
economy  is  sometimes  used  for  efficiency. 

The  efficiency  of  a  lamp  depends  on  the  temperature  at  which 
the  filament  is  run.  In  the  ordinary  lamp  this  temperature  is  between 
1,280°  and  1,330°  C,  and  the  curve  in  Fig.  4  shows  the  increase  of 
efficiency  with  the  increase  of  temperature.  The  temperature  attained 


ELECTRIC  LIGHTING  7 

by  a  filament  depends  on  the  rate  at  which  heat  is  radiated  and  the 
amount  of  power  supplied.  The  rate  of  radiation  of  heat  is  propor- 
tional to  the  area  of  the  filament,  the  elevation  in  temperature,  and 
the  emissivity  of  the  surface. 

By  emissivity  is  meant  the  number  of  heat  units  emitted  from 
unit  surface  per  degree  rise  in  temperature  above  that  of  surrounding 
bodies.  The  bright  surface  of  a  flashed  filament  has  a  lower  emis- 
sivity than  the  dull  surface  of  an  unheated  filament,  hence  less 
energy  is  lost  in  heat  radiation  and  the  efficiency  of  the  filament  is 
increased. 

As  soon  as  incandescence  is  reached,  the  illumination  increases 
much  more  rapidly  than  the  emission  of  heat,  hence  the  increase  in 


1400 


3  4  56  78  9 

Fig.  4.     Efficiency  Curve  for  Incandescent  Lamp. 


10 


efficiency  shown  in  Fig.  4.  Were  it  not  for  the  rapid  disintegration 
of  the  carbon  at  high  temperature,  an  efficiency  higher  than  3.1  watts 
could  be  obtained. 

By  a  special  treatment  of  the  carbon  filaments,  the  nature  of  the 
carbon  is  so  changed  that  the  filaments  may  be  run  at  a  higher  tem- 
perature and  the  lamps  still  have  a  life  comparable  to  that  of  the  3.1- 
watt  lamp.  Lamps  using  these  special  carbon  filaments  are  known 
as  gem  metallized  filament  lamps,  or  merely  as  gem  lamps,  and  they 
will  be  described  more  fully  later. 

Relation  of  Life  to  Efficiency.  Ordinary  Carbon  Lamp.  By 
the  useful  life  of  a  lamp  is  meant  the  length  of  time  a  lamp  will  burn 
before  its  candle-power  has  decreased  to  such  a  value  that  it  would 
be  more  economical  to  replace  the  lamp  with  a  new  one  than  to  con- 
tinue to  use  it  at  its  decreased  value.  A  decrease  to  80%  of  the  initial 
candle-power  of  carbon  lamps  is  now  taken  as  the  point  at  which  a 
lamp  should  be  replaced,  and  the  normal  life  of  a  lamp  is  in  the 


8 


ELECTRIC  LIGHTING 


neighborhood  of  800  hours.  To  obtain  the  most  economical  results, 
such  lamps  should  always  be  replaced  at  the  end  of  their  useful  life. 

In  Table  I  are  given  values  of  efficiency  and  life  of  a  3.5-watt, 
110-volt  carbon  lamp  for  various  voltages  impressed  on  the  lamp. 
These  values  are  plotted  in  Fig.  5.  The  curves  show  that  a  3% 
increase  of  voltage  on  the  lamp  reduces  the  life  by  one-half,  while  an 
increase  of  6%  causes  the  useful  life  to  fall  to  one-third  its  normal 
value.  The  effect  is  even  greater  wrhen  3.1-watt  lamps  are  used,  but 
not  so  great  with  4-watt  lamps.  From  this  we  see  that  the  regulation 
of  the  voltage  used  on  the  system  must  be  very  good  if  high  efficiency 
lamps  are  to  be  used,  and  this  regulation  will  determine  the  efficiency 
of  the  lamp  to  be  installed. 

Selection  of  Lamps.  Ordinary  Carbon  Type.  Lamps  taking  3.1 
watts  per  candle-power  will  give  satisfaction  only  when  the  regulation 
of  voltage  is  the  best — practically  a  constant  voltage  maintained  at  the 

normal  voltage  of  the  lamp. 

TABLE  I 

Effects  of  Change  in  Voltage 

Standard  3. 5- Watt  Lamp 


V  OT/TAGE 

PER  CENT.  OF 
NORMAL 

CANDLE-POWER 
PER  CENT.  OF 
NORMAL 

WATTS  PER 
CANDLE-POWER 

LIFE  PER  CENT. 
OF  NORMAL 

DETERIORATION 
PER  CENT.  OF 
NORMAL 

90 

53 

5.36 

91 

56 

5.09 

92 

61 

4.85 

93 

65 

4.63 

94 

69 

4.44 

394 

25 

95 

73 

4.26 

310 

32 

96 

78 

4.09 

247 

44 

97 

83 

3.93 

195 

51 

98 

88 

3.78 

153 

65 

99 

94 

3.64 

126 

79 

100 

100 

3.5 

100 

100 

101 

106 

3.38 

84 

118 

102 

111 

3.27 

68 

146 

103 

116 

3.16 

58 

173 

104 

123 

3.05 

47 

211 

105 

129 

2.95 

39 

253 

106 

137 

2.85 

31 

316 

107 

143 

2.76 

26 

380 

108 

152 

2.68 

21 

474 

109 

159 

2.60 

17 

575 

110 

167 

2.53 

16 

637 

Lamps  of  3.5  watts  per  candle-power  should  be  used  when  the 
regulation  is  fair,  say  with  a  maximum  variation  of  2%  from  the 
normal  voltage. 


ELECTRIC  LIGHTING 


90 


92  94  93  98  100  102  104  103  108 

Fig.  5.     Curves  of  Efficiency  and  Life  of  Carbon  Filament  Lamps. 


110 


Lamps  of  4  watts  per  candle-power  should  be  installed  when  the 
regulation  is  poor.  These  values  are  for  110-volt  lamps.  A  220- volt 
lamp  should  have  a  lower  efficiency  to  give  a  long  life.  This  is  on 


100          200  300          400  500  €00 

HOURS 
Fig.  6.     Life  Curves  of  Incandescent  Lamps. 


account  of  the  fact  that,  for  the  same  candle-power,  the  220-volt  lamp 
must  be  constructed  with  a  filament  which  is  long  and  slender  com- 
pared to  that  of  the  110-volt  lamp,  and  if  such  a  filament  is  run  at  a 
high  temperature  its  life  is  short.  The  220-volt  lamp  is  used  to  some 
considerable  extent  abroad  but  it  is  not  employed  extensively  in  the 
United  States.  It  is  customary  to  operate  such  lamps  at  an  efficiency 
of  about  4  watts  per  candle-power. 


10 


ELECTRIC  LIGHTING 


Lamps  should  always  be  renewed  at  the  end  of  their  useful  life, 
this  point  being  termed  the  smashing-point,  as  it  is  cheaper  to  replace 
the  lamp  than  to  run  it  at  the  reduced  candle-power.  Some  recom- 
mend running  these  lamps  at  a  higher  voltage,  but  that  means  at  a 
reduced  life,  and  it  is  not  good  practice  to  do  this. 


Fig.  7.     Horizontal  Distribution  Curve  for  Single-Loop  Filament. 

Fig.  6  shows  the  life  curves  of  a  series  of  incandescent  lamps. 
These  curves  show  that  there  is  an  increase  in  the  candle-power  of 
some  of  the  lamps  during  the  first  100  hours,  followed  by  a  period 
during  which  the  value  is  fairly  constant,  after  which  the  light  given 
by  the  lamp  is  gradually  reduced  to  about  80%  of  the  initial  candle- 
power. 


ELECTRIC  LIGHTING 


11 


Distribution  of  Light.  In  Fig.  1  are  shown  various  forms  of 
filaments  used  in  incandescent  lamps,  and  Figs.  7  and  8  show  the  dis- 
tribution of  light  from  a  single-loop  filament  of  cylindrical  cross- 
section.  Fig.  7  shows  the  distribution  of  light  in  a  horizontal  plane,  the 
lamp  being  mounted  in  a  vertical  position,  and  Fig.  8  shows  the  dis- 


°o 


Fig.  8.     Vertical  Distribution  Curve  for  Single-Loop  Filament. 

tribution  in  a  vertical  plane.  By  changing  the  shape  of  the  filament., 
the  light  distribution  is  varied.  A  mean  of  the  readings  taken  in 
the  horizontal  plane  forms  the  mean  horizontal  candle-power,  and 
this  candle-power  rating  is  the  one  generally  assumed  for  the  ordinary 
incandescent  lamp.  A  mean  of  the  readings  taken  in  a  vertical  plane 
gives  us  the  mean  vertical  candle-power,  but  this  value  is  of  little  use. 


12 


ELECTRIC  LIGHTING 


Mean  Spherical  Candle=Power.  When  comparing  lamps  which 
give  an  entirely  different  light  distribution,  the  mean  horizontal 
candle-power  does  not  form  a  proper  basis  for  such  comparison,  and 
the  mean  spherical  or  the  mean  hemispherical  candle-power  is  used 
instead.  By  mean  spherical  candle-power  is  meant  a  mean  value  of 
the  light  taken  in  all  directions.  The  methods  for  determining  this 
will  be  taken  up  under  photometry.  The  mean  hemispherical  candle- 
power  has  reference,  usually,  to  the  light  given  out  below  the  horizon- 
tal plane. 

The  Gem  Metallized  Filament  Lamp.  When  the  incandescent 
lamp  was  first  well  established  commercially,  the  useful  life  of  a  unit, 
when  operated  at  3.1  watts  per  candle,  was  about  200  hours.  The 
improvements  in  the  process  of  manufacture  have  been  continuous 
from  that  time  until  now,  and  the  useful  life  of  a  lamp  operated  at 
that  efficiency  to-day  is  in  the  neighborhood  of  500  hours.  Experi- 
ments in  the  treatment  of  the  carbon  filament  have  led  to  the  intro- 
duction of  the  gem  metallized  filament  lamp.  This  lamp  should  not 
be  confused  with  the  metallic  filament  lamps,  to  be  described  later, 
because  the  material  used  is  carbon,  not  a  metal.  As  a  result  of 
special  treatment  the  carbon  filament  assumes  many  of  the  character- 
istics of  a  metallic  conductor,  hence  the  term  metallized  filament. 
The  word  graphitized  has  been  proposed  in  place  of  metallized. 

TABLE  II 
*  Data  on  the  Gem  Metallized  Filament  Lamp 


WATTS 

HORIZONTAL 
C.  P. 

WATTS  PER 
CANDLE 

tSPHERICAL 

REDUCTION 
FACTOR 

$  USEFUL 
LIFE 

10 

16 

2.5 

.816 

450  hrs. 

50 

20 

2.5 

.825 

450 

80 

32 

2.5 

.816 

450 

100 

40 

2.5 

•; 

460 

125 

50 

2.5 

;; 

450 

187.5 

75 

2.5 

;; 

450 

250 

100 

2.5 

% 

450 

*  These  lamps  are  normally  rated  at  three  voltages,  114,  112,  and  110  volts,  but 
data  referring  to  the  highest  voltage  only  are  given. 

t  By  spherical  reduction  factor  is  meant  the  factor  by  which  the  horizontal  candle- 
power  must  be  multiplied  to  obtain  the  mean  spherical  candle-power. 

J  The  larger  units  are  almost  invariably  used  with  reflectors,  hence  no  spherical 
reduction  factor  is  given. 

$  The  life  of  the  lamps  when  operated  at  the  lower  voltage  is  increased  to  about 
950  hours,  and  the  efficiency  is  changed  to  2.83  watts  per  candle. 


ELECTRIC  LIGHTING 


13 


When  a  filament,  as  treated  in  the  ordinary  manner,  is  run  at  a 
high  temperature  in  a  lamp  there  is  no  improvement  of  the  filament, 
but  it  was  discovered  that,  if  the  treated  filaments  were  subjected  to 
the  extremely  high  temperature  of  the  electric  resistance  furnace — 
3,000  to  3,700  degrees  C. — at  atmospheric  pressure,  the  physical 
nature  of  the  carbon  was  changed  and  the  resulting  filament  could  be 
operated  at  a  higher  temperature  in  the  lamp  and  a  higher  efficiency, 


/J* 


30' 


IS'       30° 


Fig.  9.     Typical  Distribution  Curves  of  Gem  Lamp  with  Different  Types  of  Reflectors. 

and  still  maintain  a  life  comparable  to  that  of  a  3.1-watt  lamp.  This 
special  heating  of  the  filament,  which  is  applied  to  the  base  filament 
before  it  is  flashed,  as  well  as  to  the  treated  filament,  causes  the  cold 
resistance  of  the  carbon  to  be  very  materially  decreased  and  the  fila- 
ment, as  used  in  the  lamp,  has  a  positive  temperature  coefficient — 
rise  in  resistance  with  rise  in  temperature — a  desirable  feature  from 
the  standpoint  of  voltage  regulation  of  the  circuit  from  which  the 
lamps  are  operated.  The  high  temperature  also  results  in  the  driving 
off  of  considerable  of  the  material  which,  in  the  ordinary  lamp,  causes 
the  globe  to  blacken  after  the  lamp  has  been  in  use  for  some  time. 
The  blackening  of  the  bulb  is  responsible  to  a  considerable  degree 


14 


ELECTRIC  LIGHTING 


for  the  decrease  in  candle-power  of  the  incandescent  lamp.  The 
metallized  filament  lamp  is  operated  at  an  efficiency  of  2.5  watts  pel 
candle  with  a  useful  life  of  about  500  hours.  The  change  in  candle- 
power  with  change  in  voltage  is  less  than  in  the  ordinary  lamp  on 
account  of  the  positive  temperature  coefficient  of  the  filament.  These 
lamps  are  not  manufactured  for  very  low  candle-powers,  owing  to  the 

difficulty  of  treating  very  slender  fila- 
ments, but  they  are  made  in  sizes  con- 
suming from  40  to  250  watts.  Table  II 
gives  some  useful  information  in  connec- 
tion with  metallized  filament  lamps.  The 
filaments  are  made  in  a  variety  of  shapes 
and  the  distribution  curves  are  usually 
modified  in  practice  by  the  use  of  shades 
and  reflectors.  The  general  appearance 
of  the  lamp  does  not  differ  from  that  of 
the  ordinary  carbon  lamp.  Fig.  9  shows 
typical  distribution  curves  of  the  metallized 
filament  lamp  as  it  is  installed  in  practice. 
Metallic  Filament  Lamps.  The  Tan- 
talum Lamp.  The  first  of  the  metallic 
filament  lamps  to  be  introduced  to  any  considerable  extent  com- 
mercially was  the  tantalum  lamp.  Dr.  Bolton  of  the  Siemens  & 
Halske  Company  first  discovered  the  methods  of  obtaining  the  pure 
metal  tantalum.  This  metal  is  rendered  ductile  and  drawn  into 
slender  filaments  for  incandes- 
cent lamps.  Tantalum  has  a  high 
tensile  strength  and  high  melting 
point,  and  tantalum  filaments  are 
operated  at  temperatures  much 
higher  than  those  used  with  the 
carbon  filament  lamp.  On  ac- 
count Of  the  comparatively  low  Fig-  n-  Tantalum  Filament  Before  and 

J  After  1,000  Hours'  Use. 

specific  resistance  of  this  material 

the  filaments,  for  110-volt  lamps  must 'be  long  and  slender,  and 
this  necessitates  a  special  form  of  support.  Figs.  10,  11,  and  12 
show  some  interesting  views  of  the  tantalum  lamp  and  the  fila- 
ment. This  lamp  is  operated  at  the  efficiency  of  2  watts  per 


10. 


Hound  Bulb  Tantalum 
Lamp. 


ELECTRIC  LIGHTING 


15 


candle-power,  with  a  life  comparable  to  that  of  the  ordinary  lamp. 
By  special  treatment  it  is  possible  to  increase  the  resistance  of  the 
filaments  so  that  they  may  be  shorter  and  heavier  than  those  used  in 


Fig.  12. 

Appearance  of  Filament  After  Filament  Frame  Showing 

Having  Been  Used.  Broken  Filament. 

the  first  of  the  tantalum  lamps.  It  should  be  noted  that  the  life  of 
this  type  of  lamp  on  alternating-current  circuits  is  somewhat  uncer- 
tain; it  is  much  more  satisfactory  for  operation  on  direct-current 
circuits.  Tables  III  and  IV  give  some  general  data  on  the  tantalum 
lamp,  and  Figs.  13  and  14  show  typical  distribution  curves  for  the 
units  as  installed  at  present. 

TABLE  III 
Data  on   Tantalum  Lamp 

GENERAL  ELECTRIC  CO.,  MFTRS. 


SIZE  OF  BULB 

ESTIMATED  LIFE 

DIAMETER  OF 

BULB  IN  INCHES 

REGULAR 

ROUND 

ON  A.  C. 

ON  D.  C. 

40  watt 

2i56 

350 

800 

50     " 

2& 

350 

800 

80     " 

3i 

400 

800 

40  watt 

31 

350 

800 

80       " 

o 

400 

800 

16 


ELECTRIC  LIGHTING 


TABLE  IV 
Data  on  the  Life  of  a  25-C.  P.  Unit 


No.  OF  HOURS  BURNED 

CANDLE-POWER 

WATTS  PER  CANDLE 

0 

19.8 

2.17 

25 

23.6 

1.865 

50 

23.1 

1.90 

125 

22.3 

1.98 

225 

22.4 

1.96 

350 

22.3 

1.97 

450 

22.2 

1.98 

550 

21.2 

2.05 

650 

19.6 

2.20 

Fig.  13.      Vertical  Distribution  Curve  Without  Reflector. 

The  Tungsten  Lamp.  Following  closely  upon  the  development 
of  the  tantalum  lamp  came  the  tungsten  lamp.  Tungsten  possesses 
a  very  high  melting  point  and  an  indirect  method  is  employed  in 
farming  filaments  for  incandescent  lamps.  There  are  several  of  these 
methods  in  use.  In  one  method  a  fine  carbon  filament  is  flashed  in 
an  atmosphere  of  tungsten  oxy chloride  mixed  with  just  the  proper 
proportion  of  hydrogen,  in  which  case  the  filament  gradually  changes 


ELECTRIC  LIGHTING 


17 


to  one  of  tungsten.  A  second  method  consists  of  the  use  of  powdered 
tungsten  and  some  binding  material,  sometimes  organic  and  in  other 
cases  metallic.  The  powdered  tungsten  is  mixed  with  the  binding 
material,  the  paste  squirted  into  filaments,  and  the.  binding  material  is 
then  expelled,  usually  by  the  aid  of  heat.  Another  method  of  manu- 
facture consists  of  securing  tungsten  in  colloidal  form,  squirting  it 


Fig.  14.     Distribution  Curves  for  Tantalum  Lamp.     No.  1,  40  Watts;  No.  2,  80  Watts. 

into  filaments,  and  then  changing  them  to  the  metallic  form  by  passing 
electric  current  through  the  filaments. 

The  tungsten  lamp  has  the  highest  efficiency  of  any  of  the  com- 
mercial forms  of  metallic  filament  lamps  now  in  use,  about  1.25  watts 
per  candle-power  when  operated  so  as  to  give  a  normal  life,  and  lamps 
for  110-volt  service  and  consuming  but  40  watts  have  recently  been 
put  on  the  market.  A  25-watt  lamp  for  this  same  voltage  appears  to 
be  a  possibility.  The  units  introduced  at  first  were  of  high  candle- 
power  because  of  the  difficulty  of  manufacturing  the  slender  filaments 
required  for  the  low  candle-power  lamps. 


18 


ELECTRIC  LIGHTING 


The  advantages  of  these  metals,  tantalum  and  tungsten,  for 
incandescent  lamps  are  in  the  improved  efficiency  of  the  lamps  and 
the  good  quality  of  the  light,  white  or  nearly  white  in  both  cases. 
In  either  case  the  change  in  candle-power  with  change  in  voltage  is 
less  than  the  corresponding  change  in  an  ordinary  carbon  lamp.  The 
disadvantage  lies  in  the  fact  that  the  filaments  must  be  made  long  and 
slender,  and  hence  are  fragile,  for  low  candle-power  units  to  be  used 


Fig.  15.     Multiple  Tungsten  Lamp. 


Fig.  1  6.     Series  Tungsten  Lamp. 


on  commercial  voltages.  In  some  cases  tungsten  lamps  are  con- 
structed for  lower  voltages  and  are  used  on  commercial  circuits  through 
the  agency  of  small  step-down  transformers.  Improvements  in  the 
process  of  manufacture  of  filaments  and  of  the  method  of  their  sup- 
port have  resulted  in  the  construction  of  110-volt  lamps  for  candle- 
powers  lower  than  was  once  thought  possible.  Figs.  15  and  16  show 
the  appearance  of  the  tungsten  lamp,  and  Figs.  17  and  18  give  some 


ELECTRIC  LIGHTING 


19 


typical  distribution  curves.     Tables  V  and  VI  give  data  on  this  lamp 
as  it  is  manufactured  at  present.     One  very  considerable  application 


20* Jo 

Fig.  17.     C.  P.  Distribution  Curves  of  100- Watt  Gen.  Elec.  Tungsten 
Incandescent  Units  with  B-3,  C-3,  and  D-3  Holophanes. 

of  the  tungsten  lamp  is  to  incandescent  street  lighting  on  series  cir- 
cuits, in  which  case  the  lamp  may  be  made  for  a  low  voltage  across 
its  terminals  and  the  filament  may  be  made  comparatively  short  and 

w'o 


60° 


60* 


50°   40°  3O°  20° /O"  O"  W  20°  3O°  4O'   50" 

Fig.  18.     Candle-Power  Distribution  Given  with  40  c.  p  Gen.  Elec.  Tungsten 
Series  Lamp  and  Radial  Wave  Reflector. 

heavy.     The  tungsten  lamp  is  also  being  introduced  as  a  low  voltage 
battery  lamp. 

The  Just  lamp,  the  Z  lamp,  the  Osram  lamp,  the  Zircon- Wolfram 
lamp,  the  Osmin  lamp,  etc.,  are  all  tungsten  lamps,  the  filaments 
being  prepared  by  some  of  the  general  methods  already  described  or 
modifications  of  them. 


20 


ELECTRIC  LIGHTING 


TABLE    V 
Tungsten  Lamps 

MULTIPLE 


WATTS 

VOLTS 

CANDLE- 
POWER 

WATTS 

PER 

C.  P. 

TIP  CANDLE- 
POWER 

SPHERICAL 
REDUCTION 
FACTOR 

40 

100 

32 

1.25 

5 

76.3 

60 

125 

40 

1.25 

5.6 

76.3 

TABLE  VI 
Tungsten  Lamps 

SERIES 


AMPERES 

VOLTS 

CANDLE-POWER 

WATTS  PER  C.  P. 

4 

13.5 

40 

1.35 

20.25 

60 

5.5 

9.8 

40 

1.35 

14.7 

60 

6.6 

8.2 

40 

1.35 

12.3 

60 

7.5 

7.2 

40 

1.35 

10.8 

60 

The  Osmium  Lamp.  Very  efficient  incandescent  lamps  have 
been  constructed  using  osmium  for  the  filament.  An  indirect  method 
is  resorted  to  in  the  formation  of  these  filaments.  Osmium  lamps 
have  not  been  successful  for  commercial  voltages  because  the  fila- 
ment is  too  fragile  if  it  is  made  to  have  a  high  resistance,  so  these 
lamps  must  be  operated  in  series  or  through  the  agency  of  reducing 
transformers  if  they  are  to  be  applied  to  110-volt  circuits.  At  25 
volts,  lamps  are  constructed  giving  an  efficiency  of  about  1.5  watts  per 
candle-power  with  a  life  comparable  to  that  of  a  3.5-watt  carbon  lamp. 
Owing  to  the  introduction  of  the  tungsten  lamp,  the  osmium  lamp 
will  probably  never  be  used  to  any  great  extent. 

Other  Metallic  Filament  Lamps.  Table  VII  gives  the  melting 
points  of  several  metals  which  are  highly  refractory  and  those  already 
mentioned  are  not  the  only  ones  which  have  been  successfully  used 
in  incandescent  lamps.  Titanium,  zirconium,  iridium,  etc.,  have 
been  successfully  employed,  but  the  tantalum  and  tungsten  lamps  are 
the  only  ones  which  are  used  to  any  extent  in  the  United  States. 


ELECTRIC  LIGHTING 


21 


TABLE  VII 
Melting  Point  of  Some  Metals 


METAL 

APPROXIMATE  MELTING  POINT 
IN  DEGREES  C. 

Tungsten 

3080-3200 

Titanium 

3000 

Tantalum 

2900 

Osmium 

2500 

Platinum 

1775 

Zirconium 

1500 

Silicon 

1200 

Carbon  (not  a  metal) 

3000 

The  Helion  Lamp.  The  helion  lamp,  which  gives  considerable 
promise  of  commercial  development,  is  a  compromise  between  the 
carbon  lamp  and  the  metallic  filament  lamp.  A  slender  filament  of 
carbon  is  flashed  in  a  compound  of  silicon  (gaseous  state)  and  a  fila- 
ment composed  of  a  carbon  core  more  or 
less  impregnated  with  silicon  and  coated 
with  a  metallic  layer  is  formed.  The 
emissivity  of  such  a  filament  is  high,  the 
light  is  white  in  color,  and  the  filament  is 
strong.  The  efficiency  of  the  helion  fila- 
ment as  far  as  it  has  been  developed  is 
higher  than  that  of  a  carbon  filament 
when  operated  at  the  same  temperature. 
At  1,500  degrees  C.  the  efficiency  of  the 
helion  filament  is  2.15  watts  per  candle- 
power,  while  for  a  carbon  filament  it  is 
about  3.5  watts  per  candle-power.  Fila- 
ments of  this  type  have  been  made  which 
may  be  heated  to  incandescence  in  open 
air  without  immediate  destruction.  This 
lamp  is  not  yet  on  the  market. 

The  Nernst  Lamp.    The  Nernst  lamp 
is    still    another    form    of    incandescent 

lamp,  several  types  of  which  are  shown  in  Figs.  19,  20,  21,  and  22. 
This  lamp  uses  for  the  incandescent  material  certain  oxides  of  the 
rare  earths,  the  oxides  being  mixed  in  the  form  of  a  paste,  then 
squirted  through  a  die  into  a  string  which  is  subjected  to  a  roast- 


Fig.  19.     Westinghouse  Nernst 
Multiple-Glower  Lamp. 


22 


ELECTRIC  LIGHTING 


ing  process  forming  the  filament  or  glower  material  of  the  lamp  as 
represented  by  the  lower  white  line  in  Fig.  23.  The  more  recent 
glowers  are  made  hollow  instead  of  solid.  The  glowers  are  cut  to 

the  desired  length  and  platinum  ter- 
minals attached.  The  attachment 
of  these  terminals  to  the  glowers  is 
an  important  process  in  the  manu- 
facture of  the  lamp.  The  recent 
discovery  of  additional  oxides  has 
led  to  the  construction  of  glowers 
which  show  a  considerable  gain  in 
efficiency  over  those  previously  used. 
The  glowers  are  heated  to  incan- 
descence in  open  air,  a  vacuum  not 
being  required. 

As  the  glower  is  a  non-conductor 
when  cold,  some  form  of  heater  is 
t  necessary  to  bring  it  up  to  a  tem- 
./  p^wMA       \\  perature  at  which  it  will  conduct. 

X  r  j\t/(  r^wo   f°rms   °f   neater   nave   been 

M^,  ™  =S3Jy&          used.     One  of  them  consists   of  a 

porcelain  tube  shown  just  above 
the  glower,  Fig.  23,  about  which  a 
fine  platinum  wire  is  wound;  the 
wire  is  in  turn  coated  with  a  cement. 
Two  or  more  of  these  tubes  are 
mounted  directly  over  the  glower,  or 
glowers,  and  serve  as  a  reflector 
as  well  as  a  heater.  The  second 
form  of  heater  consists  of  a  slender 
rig.  20.  sectional  view  of  Multiple-  rod  of  refractory  material  about 

Glower  Westinghouse  Nernst  Lamp.        wm'ch    a    platinum    wire    is    WOUnd, 

the  wire  again  being  covered  with 

a  cement.  This  rod  is  then  formed  into  a  spiral  which  surrounds  the 
glower  in  the  vertical  glower  type,  or  is  formed  into  the  wafer  heater, 
Fig.  24,  now  universally  employed  in  the  Westinghouse  Nernst  lamp 
with  horizontal  glowers.  The  wafer  heater  is  bent  so  that  it  can  be 
mounted  with  several  sections  parallel  to  the  glower  or  glowers. 


ELECTRIC  LIGHTING 


23 


The  heating  device  is  connected  across  the  circuit  when  the  lamp 
is  first  turned  on,  and  it  must  be  cut  out  of  circuit  after  the  glowers 
become  conductors  in  order  to  save  the  energy  consumed  by  the 


Fig.  21.     Sectional  Views  of  Single-Glower  Westinghouse  Nernst  Lamp. 

heater  and  to  prolong  the  life  of  the  heater.  The  automatic  cut-out 
is  operated  by  means  of  an  electromagnet  so  arranged  that  current 
flows  through  this  magnet  as  soon  as  the  glower  becomes  a  conductor, 
and  contacts  in  the  heater  circuit 
are  opened  by  this  magnet.  The 
contacts  in  the  heater  circuit  are 
kept  normally  closed,  usually  by  the 
force  of  gravity. 

The  conductivity  of  the  glower 
increases  with  the  increase  of  tem- 
perature— the  material  has  a  nega- 
tive temperature  coefficient — hence 
if  it  were  used  on  a  constant  poten- 
tial circuit  directly,  the  current 
and  temperature  would  continue 
to  rise  until  the  glower  was  de- 

Fig.  22.     Westinghouse  Nernst  Screw 

stroyed.     To   prevent    the    current  Burner. 


24 


ELECTRIC  LIGHTING 


from  increasing  beyond  the  desired  value,  a  ballast  resistance  is 
used  in  series  with  the  glower.  As  is  well  known,  the  resistance  of 
iron  wire  increases  quite  rapidly  with  increase  in  temperature,  and 

the  resistance  of  a  fine  pure  iron  wire 
is  so  adjusted  that  the  resistance  of  the 
combined  circuit  of  the  glower  and  the 
ballast  becomes  constant  at  the  desired 
temperature  of  the  glower.  The  iron 
wire  must  be  protected  from  the  air 
to  prevent  oxidization  and  too  rapid 
temperature  changes,  and,  for  this 
reason,  it  is  mounted  in  a  glass  bulb 
filled  with  hydrogen.  Hydrogen  has 
been  selected  for  this  purpose  because 

Fig  23.    Westinghouse  Nernst  Screw  it  is  an  inert  gas  and  conducts  the  heat 

BurZ,;S  0"™^'       fr°m   the   ballast  to  the  walls  of  the 

Tubular  Heater.  bulb   better   than   other  gases   which 

might  be  used. 

All  of  the  parts  enumerated,  namely,  glower,  heater,  cut-out,  and 
ballast,  are  mounted  in  a  suitable  manner;  the  smaller  lamps  have  but 
one  glower  and  are  arranged  to  fit  in  an  incandescent  lamp  socket, 
while  the  larger  types  are  constructed  at  present  with  four  glowers 


Fig.  24.     Wafer  Heater  and  Mounting. 


and  are  arranged  to  be  supported  in  special  fixtures,  or  the  same  as 
small  arc  lamps.  All  parts  are  mechanically  arranged  so  that  renew- 
als may  be  easily  made  when  necessary  and  it  is  not  possible  to  insert 
a  part  belonging  to  one  type  of  lamp  into  a  lamp  of  a  different  type. 


ELECTRIC  LIGHTING 


25 


The  advantages  claimed  for  the  Nernst  Jamp  are :  High  effi- 
ciency; a  good  color  of  light;  a  good  distribution  of  light  without  the 
use  of  reflectors:  a  long  life  with  low  cost  of  maintenance;  and  a 
complete    series   of  sizes   of  units, 
thus  allowing  its  adaption  to  prac- 
tically all  classes  of  illumination.  If  | 

The  lamp  is  constructed  for 
both  direct-  and  alternating-current 
service  and  for  110  and  220  volts. 
When  the  alternating-current  lamp 
is  used  on  a  110- volt  circuit  a  small 
transformer,  commonly  called  a 
converter  coil,  Fig.  25,  is  utilized  to 
raise  the  voltage  at  the  lamp  ter- 
minals to  about  220  volts. 

Data  on  the  Nernst  lamp  in  its  present  form  are  given  in  Table 
VIII,  and  Figs.  26  and  27  show  the  form  of  distribution  curves. 

TABLE  VIII 
General  Data  on  the  Nernst  Lamp 


Fig.  25.     Converter  Coil. 


LAMP 

RATING 
IN  WATTS 

VOLTAGE 

CURRENT 

IN 

AMPERES 

MAX. 

CANDLE- 
POWER 

MEAN 
HEMISPHER- 
ICAL  C.  P. 

WATTS  PER  M.  II.  S.  c.  P. 
FROM  TEST 

66 

110 

.6 

74 

50 

1.38 

88 

220 

.4 

105 

77 

1.2 

A.C. 

1-Glower       or 

110 

110 

1.0 

131 

96.4 

1.2 

D.C. 

220 

.5 

132 

110 

1.2 

156 

114 

1.2 

220 

.6 

264 

220 

1.2 

345 

231 

1  .2      [2-Glower-i   .  -, 

396 

220 

1.8 

528 

359 

1.15    J3-Glower  [    or 

528 

220 

2.4 

745 

504 

1.09    }4-Glower  j  D-C* 

Comparison  of  the  Different  Types  of  Incandescent  Lamps.    A 

direct  comparison  of  the  different  types  of  incandescent  lamps  can- 
not be  made  but  it  is  desirable  at  this  time  to  note  the  following  points : 
The  lamps  which  are  considered  commercial  in  the  United  States 
at  the  present  time  are  the  carbon,  gem,  tantalum,  tungsten,  and 
Nernst  lamp.  The  efficiencies  ordinarily  accepted  run  in  the  order 


26 


ELECTRIC  LIGHTING 


given,  approximately  3.1,  2.5,  2, 1.25,  and  1.2  watts  per  candle  respec- 
tively. The  figure  of  1.2  watts  per  candle  for  the  Nernst  lamp  is 
based  upon  the  mean  hemispherical  candle-pov/er  and  it  should  not 
be  compared  directly  with  the  other  efficiencies.  The  color  of  the 
light  in  all  of  the  above  cases  is  suitable  for  the  majority  of  classes  of 
illumination,  the  light  from  the  higher  efficiency  units  being  some- 
what whiter  than  that  from  the  carbon  lamp.  All  of  these  lamps  are 
constructed  for  commercial  voltages  and  for  either  direct  or  alternating 
current.  The  use  of  the  tantalum  lamp  on  alternating  current  is  not 


60' 


75' 


SO'  75"  60' 

Fig.  26.     Distribution  Curve  of  132- Watt  Type  Westinghouse  Nernst  Lamp. 
Single  Glower. 

always  to  be  recommended  as  the  service  is  unsatisfactory  in  some 
cases.  The  minimum  size  of  units  for  110  volts  is  about  4  candle- 
power  for  the  carbon  lamp,  20  candle-power  for  the  metallic  filament 
lamp,  and  50  candle-power  (mean  hemispherical)  for  the  Nernst 
lamp.  Some  of  the  metallic  filament  lamps  are  constructed  for  a 
consumption  of  as  high  as  250  watts,  while  the  largest  size  of  the 
Nernst  lamp  uses  528  watts.  The  light  distribution  of  any  of  the 
units  is  subject  to  considerable  variation  through  the  agency  of  re- 
flectors, but  the  Nernst  lamp  is  ordinarily  installed  without  a  reflec- 


ELECTRIC  LIGHTING 


27 


tor.  Practically  all  of  the  other  units  of  high  candle-power  use  re- 
flectors and  only  a  few  of  the  typical  curves  of  light  distribution  curves 
with  reflectors  have  been  shown  in  connection  with  the  description 
of  the  lamps.  The  life  of  all  of  the  commercial  lamps  described  is 
considered  as  satisfactory.  The  minimum  life  is  seldom  less  than 
500  hours  and  the  useful  life  is  generally  between  500  and  1,000  hours. 
On  account  of  the  slender  filaments  employed  in  the  metallic  filament 


45- 


60°  75"  90°  75"  60° 

Fig.  27.     Distribution  of  Light  from  Multiple-Glower  "Westinghouse  Nernst  Lamps  with 

8"  Clear  Globes.     No.  1,  2  Glower;  No.  2,  3  Glower;  No.  3,  4  Glower. 

lamps  they  are  not  made  for  low  candle-powers  at  commercial  vol- 
tages. The  introduction  of  transformers  for  the  purpose  of  changing 
the  circuit  voltage  to  one  suitable  for  low  candle-power  units  has  not 
become  at  all  general  as  yet  in  this  country. 

SPECIAL    LAMPS 

The  Mercury  Vapor  Lamp.  The  mercury  vapor  lamp  in  this 
country  is  put  on  the  market  by  the  Cooper-Hewitt  Electric  Company 
and  it  is  being  used  to  a  considerable  extent  for  industrial  illumination. 
In  this  lamp  mercury  vapor,  rendered  incandescent  by  the  passage 
of  an  electric  current  through  it,  is  the  source  of  light.  In  its  standard 
form  this  lamp  consists  of  a  long  glass  tube  from  which  the  air  has 
been  carefully  exhausted,  and  which  contains  a  small  amount  of 
metallic  mercury.  The  mercury  is  held  in  a  large  bulb  at  one  end  of 


28  ELECTRIC  LIGHTING 

the  tube  and  forms  the  negative  electrode  in  the  direct-current  lamp. 
The  other  electrode  is  formed  by  an  iron  cup  and  the  connections 
between  the  lamp  terminals  and  the  electrodes  are  of  platinum  where 
this  connection  passes  through  the  glass.     Fig.  28  gives  the  general 
appearance  of  a  standard  lamp  having  the  following  specifications: 
Total  watts  (110  volts,  3.5  amperes)  =  385 
Candle-power  (M.  H.  with  reflector)  =  700 
Watts  per  candle  =  0.55 
Length  of  tube,  total  =  55  in. 
Length  of  light-giving  section  =  45  in. 
Diameter  of  tube  =  1  in. 

Height  from  lowest  point  of  lamp  to  ceiling  plate  =  22  in. 
For  220-volt  service  two  lamps  are  connected  in  series. 
The  mercury  vapor,  at  the  start,  may  be  formed  in  two  ways: 
First,  the  lamp  may  be  tipped  so  that  a  stream  of  mercury  makes 

contact  between  the  two  elec- 
trodes and  mercury  is  vaporized 
when  the  stream  breaks.  Second, 
by  means  of  a  high  inductance 
and  a  quick  break  switch,  a  very 
high  voltage  sufficient  to  pass  a 
current  from  one  electrode  to  the 

Fig.  28.    Cooper-Hewitt  Mercury  Vapor  ,1         ,1  1,1  •     » 

Lamp  other  through  the  vacuum,  is  in- 

duced and  the  conducting  vapor 

is  formed.  The  tilting  method  of  starting  is  preferred  and  this 
tilting  is  brought  about  automatically  in  the  more  recent  types  of 
lamp  Fig.  29  shows  the  connections  for  automatically  starting  two 
lamps  in  series.  A  steadying  resistance  and  reactance  are  connected 
as  shown  in  this  figure. 

The  mercury  vapor  lamp  is  constructed  in  rather  large  units, 
the  55-volt,  3.5-ampere  lamp  being  the  smallest  standard  size.  The 
color  of  the  light  emitted  is  objectionable  for  some  purposes  as  there 
is  an  entire  absence  of  red  rays  and  the  light  is  practically  monochro- 
matic. The  illumination  from  this  type  of  lamp  is  excellent  where 
sharp  contrast  or  minute  detail  is  to  be  brought  out,  and  this  fact 
has  led  to  its  introduction  for  such  classes  of  lighting  as  silk  mills  and 
cotton  mills.  On  account  of  its  color  the  application  of  this  lamp  is 
limited  to  the  lighting  of  shops,  offices,  and  drafting  rooms,  or  to  disc 


ELECTRIC  LIGHTING 


29 


play  windows  where  the  goods  shown  will  not  be  changed  in  appear- 
ance by  the  color  of  the  light.  It  is  used  to  a  considerable  extent  in 
photographic  work  on  account  of  the  actinic  properties  of  the  light. 
Special  reactances  must  be  provided  for  a  mercury  arc  lamp  operating 
on  single-phase,  alternating-current  circuits. 

The  Moore  Tube  Light.  The  Moore  light  makes  use  of  the 
familiar  Geissler  tube  discharge — discharge  of  electricity  through  a 
vacuum  tube — as  a  source  of  illumination.  The  practical  application 
of  this  discharge  to  a  system  of  lighting  has  involved  a  large  amount 


t03-I2O  volts 


3.5  Ampere 


Switch 


H  Inductance 

\ 
j 

I|LKJ  |  induct  once 
1  Coil 

==#^J 
Fig.  29.     Wiring  Diagram.     Two  H  Automatic  Lamps  in  Series. 

of  consistent  research  on  the  part  of  the  inventor  and  it  has  now  been 
brought  to  such  a  stage  that  several  installations  have  been  made. 
The  system  has  many  interesting  features. 

In  the  normal  method  of  installation,  a  glass  tube  If  inches  in 
diameter  is  made  up  by  connecting  standard  lengths  of  glass  tubing 
together  until  the  total  desired  length  is  reached,  and  this  continuous 
tube,  which  forms  the  source  of  light  when  in  operation,  is  mounted 
in  the  desired  position  with  respect  to  the  plane  of  illumination.  In 
many  cases  the  tube  forms  a  large  rectangle  mounted  just  beneath 
the  ceiling  of  the  room  to  be  lighted.  The  tube  may  be  of  any  reason- 
able length,  actual  values  running  from  40  to  220  feet.  In  order  to 


30 


ELECTRIC  LIGHTING 


Tube  distributed,  in 
cmy  form  des~ire<4  to 
ofsoofeet. 


provide  an  electrical  discharge  through  this  tube  it  is  customary  to 
lead  both  ends  of  the  tube  to  the  high  tension  terminals  of  a  trans- 
former, the  low  tension  side  of  which  may  be  connected  to  the  alter- 
nating-current lighting  mains.  This  transformer  is  constructed  so 
that  the  high  tension  terminals  are  not  exposed  and  the  current  is 
led  into  the  tube  by  means  of  platinum  wires  attached  to  carbon 
electrodes.  The  electrodes  are  about  eight  inches  in  length.  The 
ends  of  the  tube  and  the  high  tension  terminals  are  enclosed  in  a  steel 
casing  so  as  to  effectually  prevent  anything  from  coming  in  contact 
with  the  high  potential  of  the  system.  As  stated,  the  low  tension  side 

of  the  trans- 
former is  con- 
nected to  the 
usual  60-cycle 
lighting  mains. 
If  direct  current 
is  used  for  distribution,  a  motor- 
generator  set  for  furnishing  alter- 
nating current  to  the  primary  of 
the  transformer  is  required.  Any 
frequency  from  60  cycles  up  is 
suitable  for  the  operation  of  these 
tubes.  At  lower  frequencies  there 
is  some  appreciable  variation  of 
the  light  emitted.  One  other  de- 
vice is  necessary  for  the  suitable 
operation  of  this  form  of  light  and 
this  is  known  as  the  regulator.  In  order  to  maintain  a  constant  pres- 
sure inside  the  tube,  and  such  a  constant  pressure  is  necessary  for 
its  satisfactory  operation,  there  must  be  some  automatic  device  which 
will  allow  a  small  amount  of  gas  to  enter  the  tube  at  intervals  while 
it  is  in  operation.  The  regulator  accomplishes  this  purpose.  Fig. 
30  shows  a  diagram  of  the  very  simple  connections  of  the  system  and 
gives  the  relative  positions  occupied  by  the  transformer,  tube,  and  regu- 
lator. Fig.  31  gives  an  enlarged  view  of  the  regulator,  a  description 
of  which  and  its  method  of  operation  is  given  as  follows: 

A  piece  of  £-inch  glass  tubing  is  supported  vertically  and  its  bottom  end 
is  contracted  into  a  f -inch  glass  tube  which  extends  to  the  main  lighting  tube. 


Fig.    30.      Diagram     Showing     Essentia 
Features  of  the  Moore  Light.     1 .  Light- 
ing Tube;    2.    Transformer    Case; 
3.  Lamp  Terminals;    4.  Trans- 
former; 5,  6,  7,  8,  Regulators. 


ELECTRIC  LIGHTING 


31 


At  the  point  of  contraction  at  the  bottom  of  the  f-inch  tube  there  is  sealed 
by  means  of  cement  a  |— inch  carbon  plug,  the  porosity  of  which  is  not  great 
enough  to  allow  mercury  to  percolate  through  it  but  which  will  permit  gases 
easily  to  pass,  due-  to  the  high  vacuum  of  the 
lighting  tube  connected  to  the  lower  end  of  the 
plug,  and  approximately  atmospheric  pressure 
above  it.  This  carbon  plug  is  normally  com- 
pletely covered  with  what  would  correspond  to 
a  thimbleful  of  mercury  which  simply  seals  the 
pores  of  the  carbon  plug,  and  therefore  has 
nothing  whatever  to  do  with  the  conducting 
properties  of  the  gas  in  the  main  tube  which 
produces  the  light.  Partly  immersed  in  the 
mercury  and  concentric  with  the  carbon  plug, 
is  another  smaller  and  movable  glass  tube,  the 
upper  end  of  which  is  filled  with  soft  iron  wire, 
which  acts  as  the  core  of  a  small  solenoid  con- 
nected in  series  with  the  transformer.  The 
action  of  the  solenoid  is  to  lift  the  concentric 
glass  tube  partly  out  of  the  mercury,  the  sur- 
face of  which  falls  and  thereby  causes  the 
minute  tip  of  the  conical  shaped  carbon  plug 
to  be  slightly  exposed  for  a  second  or  two. 

This  exposure  is  sufficient  to  allow 
a  small  amount  of  gas  to  enter  the  tube, 
the  current  decreases  slightly,  and  the 
carbon  plug  is  again  sealed.  The  process 
above  described  takes  place  at  intervals 
of  about  one  minute  when  the  tube  is  in 
operation. 

The  color  of  the  light  emitted  by  the 
tube  depends  upon  the  gas  used  in  it. 
The  regulator  is  fitted  with  some  chem- 
ical arrangement  whereby  the  proper  gas 
is  admitted  to  it  when  the  tube  is  in  opera- 
tion. Nitrogen  is  employed  when  the  tube 
gives  the  highest  efficiency  and  the  light 
emitted  when  this  gas  is  used  is  yellowish 
in  color.  Air  gives  a  pink  appearance  to 
the  tube  and  carbon  dioxide  is  employed  when  a  white  light  is  desired. 

Table  IX  gives  general  data  on  the  Moore  tube  light.  The 
advantages  claimed  for  this  light  are:  High  efficiency,  good  color,  and 
low  intrinsic  brilliancy. 


Fig.  31.     Regulating  Valve. 


32 


ELECTRIC  LIGHTING 


TABLE  IX 
Data  on  the  Moore  Tube  Light 


LENGTH  OF 
TUBE 

TRANSFORMER 
CAPACITY 

POWER  FACTOR 
OF  CIRCUIT 

VOLTAGE  AT  LAMP  TERMINALS 

40-70   ft. 

80-125  " 

2        kw. 
2.75  " 

65-84% 

3,146  for  40-ft.  tube,  at 
12  Hefners  per  ft. 

130-180  " 

3.5     " 

190-220  " 

4.5     " 

12,441  for  220-ft.  tube,  at 
12  hefners  per  ft. 

Pressure  in  tube,  about  ^  mm.  of  mercury. 

Watts  per  hefner,  3.2  for    20-foot  tube  including  transformer. 

Watts  per  hefner,  1.4  for  180-foot  tube  including  transfo.'mer. 

Hefner  per  foot,  normal,  12. 

Note  that  one  hefner  equals  0.88  candle-power. 


ARC  LAMPS 

The  Electric  Arc.  Suppose  two  carbon  rods  are  connected  in 
an  electric  circuit,  and  the  circuit  closed  by  touching  the  tips  of  these 
rods  together;  on  separating  the  carbons  again  the  circuit  will  not 
be  broken,  provided  the  space  between  the  carbons  be  not  too  great, 

but  will  be  maintained  through  the  arc 
formed  at  these  points.  This  phenom- 
enon, which  is  the  basis  of  the  arc 
light,  was  first  observed  on  a  large  scale 
by  Sir  Humphrey  Davy,  who  used  a 
battery  of  2,000  cells  and  produced  an 
arc  between  charcoal  points  four  inches 
apart. 

As  the  incandescence  of  the  carbons 
across  which  an  arc  is  maintained,  to- 
gether with  the  arc  itself,  forms  the 
source  of  light  for  a  large  portion  of  arc 
lamps,  it  will  be  well  to  study  the 
nature  of  the  arc.  Fig.  32  shows  the 
general  appearance  of  an  arc  between  two  carbon  electrodes  when 
maintained  by  direct  current. 


Fig.  32.      The  Electric  Arc  between 
Carbon  Terminals. 


ELECTRIC  LIGHTING 


33 


Here  the  current  is  assumed  as  passing  from  the  top  carbon  to 
the  bottom  one  as  indicated  by  the  arrow  and  signs.  We  find,  in  the 
direct-current  arc,  that  the  most  of  the  light  issues  from  the  tip  of  the 
positive  carbon,  or  electrode,  and  this  portion  is  known  as  the  crater 
of  the  arc.  This  crater  has  a  temperature  of  from  3,000°  to  3,500°  C., 
the  temperature  at  which  the  carbon  vaporizes,  and  gives  fully  80  to 
85%  of  the  light  furnished  by  the  arc.  The  negative  carbon  becomes 
pointed  at  the  same  time  thai  the  positive  one  is  hollowed  out  to  form 
the  crater,  and  it  is  also  incandescent  but  not  to  as  great  a  degree  as 
the  positive  carbon.  Between  the  electrodes  there  is  a  band  of  violet 
light,  the  arc  proper,  and  this 
is  surrounded  by  a  luminous 
zone  of  a  golden  yellow  color. 
The  arc  proper  does  not  fur- 
nish more  than  5%  of  the  light 
emitted  when  pure  carbon 
electrodes  are  used. 

The  carbons  are  worn 
away  or  consumed  by  the 
passage  of  the  current,  the 
positive  carbon  being  con- 
sumed about  twice  as  rapidly 
as  the  negative. 

The  light  distribution 
curve  of  a  direct-current  arc, 
taken  in  a  vertical  plane,  is 
shown  in  Fig.  33.  Here  it  is  seen  that  the  maximum  amount  of  light 
is  given  off  at  an  angle  of  about  50°  from  the  vertical,  the  negative 
carbon  shutting  off  the  rays  of  light  that  are  thrown  directly  down- 
ward from  the  crater. 

If  alternating  current  is  used,  the  upper  carbon  becomes  positive 
and  negative  alternately,  and  there  is  no  chance  for  a  crater  to  be 
formed,  both  carbons  giving  off  the  same  amount  of  light  and  being 
consumed  at  about  the  same  rate.  The  light  distribution  curve  of 
an  alternating-current  arc  is  shown  in  Fig.  34. 

AroLamp  Mechanisms.  In  a  practical  lamp  we  must  have  not 
only  a  pair  of  carbons  for  producing  the  arc,  but  also  means  for  sup- 
porting these  carbons,  together  with  suitable  arrangements  for  leading 


Fig.  33. 


Distribution  Curve  for  D.  C.  Arc 
Lamp  (Vertical  Plane). 


34 


ELECTRIC  LIGHTING 


the  current  to  them  and  for  maintaining  them  at  the  proper  distance 
apart.  The  carbons  are  kept  separated  the  proper  distance  by  the 
operating  mechanisms  which  must  perform  the  following  functions: 

1.  The  carbons  must  be  in  contact,  or  be  brought  into  contact,  to  start 
the  arc  when  the  current  first  flows. 

2.  They  must  be  separated  at  the  right  distance  to  form  a  proper  arc 
immediately  afterward. 


Fig.  34.     Distribution  Curve  for  A.  C.  Arc  Lamp  (Vertical  Plane). 

3.  The  carbons  must  be  fed  to  the  arc  as  they  are  consumed. 

4.  The  circuit  should  be  open  or  closed  when  the  carbons  are  entirely 
consumed,  depending  on  the  method  of  power  distribution. 

The  feeding  of  the  carbons  may  be  done  by  hand,  as  is  the  case 
in  some  stereopticons  using  an  arc,  but  for  ordinary  illumination  the 
striking  and  maintaining  of  the  arc  must  be  automatic.  It  is  made 
so  in  all  cases  by  means  of  solenoids  acting  against  the  force  of  gravity 
or  against  springs.  There  are  an  endless  number  of  such  mechanisms, 


ELECTRIC  LIGHTING 


35 


but  a  few  only  will  be  described  here.     They  may  be  roughly  divided 
into  three  classes: 

1.  Shunt  mechanisms. 

2.  Series  mechanisms. 

3.  Differential  mechanisms. 

Shunt  Mechanisms.  In  shunt  lamps,  the  carbons  are  held  apart 
before  the  current  is  turned  on,  and  the  circuit  is  closed  through  a 
solenoid  connected  in  across  the 
gap  so  formed.  All  of  the  cur- 
rent must  pass  through  this  coil 
at  first,  and  the  plunger  of  the 
solenoid  is  arranged  to  draw  the 
carbons  together,  thus  starting 
the  arc.  The  pull  of  the  solenoid 
and  that  of  tha  springs  are  ad- 
justed to  maintain  the  arc  at  its 
proper  length. 

Such  lamps  have  the  disad- 
vantage of  a  high  resistance  at 
the  start — 450  ohms  or  more — 
and  are  difficult  to  start  on  series 
circuits,  due  to  the  high  voltage 
required.  They  tend  to  maintain 
a  constant  voltage  at  the  arc,  but 
do  not  aid  the  dynamo  in  its 
regulation,  so  that  the  arcs  are 
liable  to  be  a  little  unsteady. 

Series  Mechanisms.  With 
the  series-lamp  mechanism,  the 
carbons  are  together  when  the  lamp  is  first  started  and  the  current, 
flowing  in  the  series  coil,  separates  the  electrodes,  striking  the  arc. 
When  the  arc  is  too  long,  the  resistance  is  increased  and  the  current 
lowered  so  that  the  pull  of  the  solenoid  is  weakened  and  the  carbons 
feed  together.  This  type  of  lamp  can  be  used  only  on  constant- 
potential  systems. 

Fig.  35  shows  a  diagram  of  the  connection  of  such  a  lamp.  This 
diagram  is  illustrative  of  the  connection  of  one  of  the  lamps  manu- 
factured by  the  Western  Electric  Company,  for  use  on  a  direct-current, 


Fig.  35.    Series  Mechanism  for  D.  C. 
Arc  Lamp. 


36 


ELECTRIC  LIGHTING 


constant-potential  system.  The  symbols  +  and  —  refer  to  the  termi- 
nals of  the  lamp,  and  the  lamp  must  be  so  connected  that  the  current 
flows  from  the  top  carbon  to  the  bottom  one.  R  is  a  series  resistance, 
adjustable  for  different  voltages  by  means  of  the  shunt  G.  F  and  D 
are  the  controlling  solenoids  connected  in  series  with  the  arc.  B  and 
C  are  the  positive  and  negative  carbons  respectively,  while  A  is  the 
switch  for  turning  the  current  on  and  off.  H  is  the  plunger  of  the 

solenoids  and  I  the  carbon  clutch, 
^  this  being  what  is  known  as  a 
carbon-feed  lamp.  The  carbons 
are  together  when  A  is  first  closed, 
the  current  is  excessive,  and  the 
plunger  is  drawn  up  into  the  so- 
lenoids, lifting  the  carbon  B  until 
the  resistance  of  the  arc  lowers  the 
current  to  such  a  value  that  the 
pull  of  the  solenoid  just  counter- 
balances the  weight  of  the  plunger 
and  carbon.  G  must  be  so  adjusted 
that  this  point  is  reached  when  the 
arc  is  at  its  normal  length. 

Differential  Mechanisms.  In 
the  differential  lamp,  the  series  and 
shunt  mechanisms  are  combined, 
the  carbons  being  together  at  the 
start,  and  the  series  coil  arranged 
so  as  to  separate  them  while  the 
shunt  coil  is  connected  across  the 
arc,  as  before,  to  prevent  the  carbons  from  being  drawn  too  far  apart. 
This  lamp  operates  only  over  a  low-current  range,  but  it  tends  to  aid 
the  generator  in  its  regulation. 

Fig.  36  shows  a  lamp  having  a  differential  control,  this  also  being 
the  diagram  of  a  Western  Electric  Company  arc  lamp  for  a  direct- 
current,  constant-potential  system.  Here  S  represents  the  shunt  coil 
and  M  the  series  coil,  the  armature  of  the  two  magnets  A  and  A'  being 
attached  to  a  bell-crank,  pivoted  at  B,  and  attached  to  the  carbon 
clutch  C.  The  pull  of  coil  S  tends  to  lower  the  carbon  while  that  of 
M  raises  the  carbon,  and  the  two  are  so  adjusted  that  equilibrium  is 


Fig.  36. 


Differential  Mechanism  for 
D.  C.  Arc  Lamp. 


ELECTRIC  LIGHTING 


37 


reached  when  the  arc  is  of  the  proper  length.  All  of  the  lamps  are 
fitted  with  an  air  dashpot,  or  some  damping  device,  to  prevent  too 
rapid  movements  of  the  working  parts. 

The  methods  of  supporting  the  carbons  and  feeding  them  to 
the  arc  may  be  divided  into  two  classes: 

1.  Rod-feed  mechanism. 

2.  Carbon-feed  mechanism. 

Rod-Feed  Mechanism. 
Lamps  using  a  rod  feed  have 
the  upper  carbons  supported 
by  a  conducting  rod,  and  the 
regulating  mechanism  acts  on 
this  rod,  the  current  being  fed 
to  the  rod  by  means  of  a  sliding 
contact.  Fig.  37  shows  the  ar- 
rangement of  this  type  of  feed. 
The  rod  is  shown  at  R,  the 
sliding  contact  at  B,  and  the 
carbon  is  attached  to  the  rod 
atC. 

These  lamps  have  the  ad- 
vantage that  carbons,  which 
do  not  have  a  uniform  cross- 
section  or  smooth  exterior,  may 
be  used,  but  they  possess  the 
disadvantage  of  being  very 
long  in  order  to  accommodate 
the  rod.  The  rod  must  also  be 
kept  clean  so  as  to  make  a 
good  contact  with  the  brush. 

Carbon-Feed  Mechanism.  In  carbon-feed  lamps  the  controlling 
mechanism  acts  on  the  carbons  directly  through  some  form  of  clutch 
such  as  is  shown  at  C  in  Fig.  38.  This  clamp  being  lifted  grips  the 
carbon,  but  allows  the  carbon  to  slip  through  it  when  the  tension 
is  released.  For  this  type  of  feed  the  carbon  must  be  straight  and 
have  a  uniform  cross-section  as  well  as  a  smooth  exterior.  The 
current  may  be  led  to  the  carbon  by  means  of  a  flexible  lead  and  a 
short  carbon  holder 


Fig.  37.     Rod-Feed  Mechanism. 


38 


ELECTRIC  LIGHTING 


TYPES  OF  ARC  LAMPS 

Arc  lamps  are  constructed  to  operate  on  direct-current  or  alter- 
nating-current systems  when  connected  in  series  or  in  multiple.  They 
are  also  made  in  both  the  open  and  the  enclosed  forms. 

By  an  open  arc  is  meant  an  arc  lamp  in  which  the  arc  is  exposed 
to  the  atmosphere,  while  in  the  enclosed  arc  an  inner  or  enclosing 


Fig.  38.     Enclosed  Arc  Lamp  with  Carbon  Feed  Mechanism. 

globe  surrounds  the  arc,  and  this  globe  is  covered  with  a  cap  which 
renders  it  nearly  air-tight.  Fig.  38  is  a  good  example  of  an  enclosed 
arc  as  manufactured  by  the  General  Electric  Company. 

Direct=Current  Arcs.  Open  Types  of  Arcs  for  direct-current 
systems  were  the  first  to  be  used  to  any  great  extent.  When  used 
they  are  always  connected  in  series,  and  are  run  from  some  form  of 


ELECTRIC  LIGHTING  39 

special  arc  machine,  a  description  of  which  may  be  found  in  "Types 
of  Dynamo  Electric  Machinery." 

Each  lamp  requires  in  the  neighborhood  of  50  volts  for  its  opera- 
tion, and,  since  the  lamps  are  connected  in  series,  the  voltage  of  the 
system  will  depend  on  the  number  of  lamps;  therefore,  the  number 
of  lamps  that  may  be  connected  to  one  machine  is  limited  by  the 
maximum  allowable  voltage  on  that  machine.  By  special  construction 
as  many  as  125  lamps  are  run  from  one  machine,  but  even  this  size 
of  generator  is  not  so  efficient  as  one  of  greater  capacity.  Such  gen- 
erators are  usually  wound  for  6.6  or  9.6  amperes.  Since  the  carbons 
are  exposed  to  the  air  at  the  arc,  they  are  rapidly  consumed,  requiring 
that  they  be  renewed  daily  for  this  type  of  lamp. 

Double-carbon  arcs.  In  order  to  increase  the  life  of  the  early 
form  of  arc  lamp  without  using  too  long  a  carbon,  the  double-carbon 
type  was  introduced.  This  type  uses  two  sets  of  carbons,  both  sets 
being  fed  by  one  mechanism  so  arranged  that  when  one  pair  of  the  elec- 
trodes is  consumed  the  other  is  put  into  service.  At  present  nearly 
all  forms  of  the  open  arc  lamp  have  disappeared  on  account  of  the 
better  service  rendered  by  the  enclosed  arc. 

Enclosed  arcs  for  series  systems  are  constructed  much  the  same 
as  the  open  lamp,  and  are  controlled  by  either  shunt  or  differential 
mechanism.  They  require  a  voltage  from  68  to  75  at  the  arc,  and  are 
usually  constructed  for  from  5  to  6.8  amperes.  They  also  require  a 
constant-current  generator  or  a  rectifier  outfit  if  used  on  alternating- 
current  circuits. 

Constant-potential  arcs  must  have  some  resistance  connected  in 
series  with  them  to  keep  the  voltage  at  the  arc  at  its  proper  value. 
This  resistance  is  made  adjustable  so  that  the  lamps  may  be  used  on 
any  circuit.  Its  location  is  clearly  shown  in  Fig.  38,  one  coil  being 
located  above,  the  other  below  the  operating  solenoids. 

Alternating-Current  Arcs.  These  do  not  differ  greatly  in  con- 
struction from  the  direct-current  arcs.  When  iron  or  other  metal 
parts  are  used  in  the  controlling  mechanism,  they  must  be  laminated 
or  so  constructed  as  to  keep  down  induced  or  eddy  currents  which 
might  be  set  up  in  them.  For  this  reason  the  metal  spools,  on  which 
the  solenoids  are  wound,  are  slotted  at  some  point  to  prevent  them 
from  forming  a  closed  secondary  to  the  primary  formed  by  the  solen- 
oid winding.  On  constant-potential  circuits  a  reactive  coil  is  used 


40 


ELECTRIC  LIGHTING 


in  place  of  a  part  of  the  resistance  for  cutting  down  the  voltage  at  the 
arc. 

Interchangeable  Arc.  Interchangeable  arcs  are  manufactured 
which  may  be  readily  adjusted  so  as  to  operate  on  either  direct  or 
alternating  current,  and  on  voltages  from  110  to  220.  Two  lamps 
may  be  run  in  series  on  220-volt  circuits. 

The  distribution  of  light,  and  the  resulting  illumination  for  the 
different  lamps  just  considered,  will  be  taken  up  later.  Aside  from 
the  distribution  and  quality  of  light,  the  enclosed  arc  has  the  advan- 
tage that  the  carbons  are  not  consumed  so  rapidly  as  in  the  open  lamp 
because  the  oxygen  is  soon  exhausted  from  the  inner  globe  and  the 
combustion  of  the  carbon  is  greatly  decreased.  They  will  burn 
from  80  to  100  hours  without  retrimming. 

TABLE  X 
Rating  of  Enclosed  Arcs 


WATTS  CONSUMED 

MEAN   INTENSITY 
IN   H.  U. 

MEAN  WATTS 

Oi 

a 

a 

2 

SPHERICAL 

aj  ,  M 
^.  S  a 

SPHERICAL 
H.  U. 

^  i  ^ 

•3 

H 

(X 

a 

o 

M 

a 

ti 

llsi 

« 

«g 

MK  £  d 

0 
P. 

£ 
& 
0 

M 

A 

H 

00 

CO 

CLEAR 
OUTER 

00 

oo 

CLEAR 
OUTER 

235 

332 

2.37 

1.66 

1 

5.01 

551 

401 

150 

172 

256* 

362* 

3.10 

2.18* 

1  .52* 

3 

5.08 

559 

406 

252 

195 

216 

282 

2.85 

2.60 

1.99 

4 

4.76 

524 

381 

143 

127 

139 

208 

4.12 

3.76 

2.52 

5 

4.16f 

458 

333 

125 

154 

174 

221 

2.96 

2.63 

2.07 

7 

4.76 

524 

381 

143 

203 

333 

317 

2.63 

2.20 

1.65 

9 

4.84 

532 

387 

145 

182 

226 

281 

2.83 

2.38 

1.89 

10 

4  99 

549 

399 

150 

202 

242 

309 

2.74 

2   24 

1  .77 

12 

4.87 

536 

3CO 

146 

178 

195 

230 

3.05 

2^66 

2.33 

Mean 

4.9 

529 

384 

144 

176 

207 

272 

3.03 

2.60 

1.98 

a 

1 

Jj 

H 

cu 

P5 

o 

tf 

55 

W 

« 

•"i  O 

« 

i  O 

M 

d 

tf 

PH 

j 

*  o  S 

£  o  u 

0 

^ 

0  •<  •«! 

£ 

0  <!  PS 

w 

•< 

0 

I—  I 

M 

5 

101 

6.40 

448 

.63 

340 

.82 

108 

127 

141 

206 

3.52 

3.17 

2.17 

203 

236 

2.26 

1  .94 

102 

6.79 

459 

.61 

375 

.73 

84 

146 

176f 

226t 

3.31 

2.60t 

1.72t 

103 

5.89 

424 

.65 

344 

.75 

80 

116 

130 

147 

3.66 

3.15 

2.88 

105 

6.20 

414 

.61 

382 

.80 

32 

128 

187 

219 

3.24 

2.20 

1.89 

153 

169 

2.56 

2.23 

106 

6.12 

378 

.56 

298 

.70 

80 

132 

182t 

284 

2.82 

2.19t 

1.48f 

108 

6.48 

457 

.64 

383 

.80 

74.5 

133 

175 

211 

3.20 

2.61 

2.16 

110 

6.18 

339 

.49 

276 

.72 

63 

140* 

126 

143 

2.41* 

2.68 

2.37 

Mean 

6.29    417 

.60 

342 

.76 

74.5 

130 

159 

190 

3.31 

2.66 

2.23 

*Condition  of  no  outer  globe.     fCondition  with  shade  on  lamp.    H.U.  Hefner  Units. 


Rating  of  Arc  Lamps. 

follows : 


Open  arcs  have  been  classified  as 


ELECTRIC  LIGHTING  41 

Full  Arcs,  2,000  candle-power  taking  9.5  to  10  amps,  or  450-480  watts. 
Half  Arcs,  1,200  candle-power  taking  6.5  to  7  amps,  or  325-350  watts. 

These  candle-power  ratings  are  much  too  high,  and  run  more 
nearly  1,200  and  700,  respectively,  for  the  point  of  maximum  intensity 
and  less  than  this  if  the  mean  spherical  candle-power  be  taken.  For 
this  reason,  the  ampere  or  watt  rating  is  now  used  to  indicate  the 
power  of  the  lamp.  It  is  now  recommended  that  specifications  for 
street  lighting  should  be  based  upon  the  illumination  produced.  This 
point  is  considered  later  under  the  topic  of  street  lighting.  Enclosed 
arcs  use  from  3  to  6.5  amperes,  but  the  voltage  at  the  arc  is  higher 
than  for  the  open  lamp.  Table  X  gives  some  data  on  enclosed  arcs 
on  constant-potential  circuits. 

Efficiency.     The  efficiency  oe  arc  lamps  is  given  as  follows: 

Direct-Current  Arc  (enclosed)  2.9  watts  per  candle-power. 
Alternating-Current  Arc  (enclosed)  2.95  watts  per  candle-power. 
Direct-Current  Arc  (open)  .6-1.25  watts  per  candle-power. 

Carbons  for  Arc  Lamps.  Carbons  are  either  moulded  or  forced 
from  a  product  known  as  petroleum  coke  or  from  similar  materials 
such  as  lampblack.  The  material  is  thoroughly  dried  by  heating  to  a 
high  temperature,  then  ground  to  a  find  powder,  and  combined  with 
some  substance  such  as  pitch  which  binds  the  fine  particles  of  carbon 
together.  After  this  mixture  is  again  ground  it  is  ready  for  moulding. 
The  powder  is  put  in  steel  moulds  and  heated  until  it  takes  the  form 
of  a  paste,  when  the  necessary  pressure  is  applied  to  the  moulds.  For 
the  forced  carbons,  the  powder  is  formed  into  cylinders  which  are 
placed  in  machines  which  force  the  material  through  a  die  so  arranged 
as  to  give  the  desired  diameter.  The  forced  carbons  are  often  made 
with  a  core  of  some  special  material,  this  core  being  added  after  the 
carbon  proper  has  been  finished.  The  carbons,  whether  moulded 
or  forced,  must  be  carefully  baked  to  drive  off  all  volatile  matter. 
The  forced  carbon  is  always  more  uniform  in  quality  and  cross- 
section,  and  is  the  type  of  carbon  which  must  be  used  in  the  carbon- 
feed  lamp.  The  adding  of  a  core  of  a  different  material  seems  to 
change  the  quality  of  light,  and  being  more  readily  volatilized,  keeps 
the  arc  from  wandering. 

Plating  of  carbons  with  copper  is  sometimes  resorted  to  for 
moulded  forms  for  the  purpose  of  increasing  the  conductivity,  and, 
by  protecting  the  carbon  near  the  arc*  prolonging  the  life. 


42  ELECTRIC  LIGHTING 

The  Flaming  Arc.  In  the  carbon  arc  the  arc  proper  gives  out 
but  a  small  percentage  of  the  total  amount  of  light  emitted.  In  order 
to  obtain  a  light  in  which  more  of  the  source  of  luminosity  is  in  the 
arc  itself,  experiments  have  been  made  with  the  use  of  electrodes  im- 
pregnated with  certain  salts,  as  well  as  with  electrodes  of  a  material 
different  than  carbon.  The  result  of  these  experiments  has  been  to 
place  upon  the  market  the  flaming  arc  lamps  and  the  luminous  arc 
lamps — lamps  of  high  candle-power,  good  efficiency,  and  giving  vari- 
ous colors  of  light.  These  lamps  may  be  put  in  two  classes :  One  class 
uses  carbon  electrodes,  these  electrodes  being  impregnated  with  certain 

salts  wThich  add  luminosity  to  the 
arc,  or  else  fitted  with  cores  which 
contain  the  required  material; 
the  other  class  covering  lamps 
which  do  not  employ  carbon,  the 
most  notable  example  being  the 
magnetite  arc  which  uses  a  copper 
segment  as  one  electrode  and  a 
magnetite  stick  as  the  other 
electrode. 

Flaming  arcs  of  the  first  class 
Fig.  39.    Diagram  of  Bremer  Flaming  Arc.     are  made  in  two  general  types: 

One  in  which  the  electrodes  are 

placed  at  an  angle,  and  the  other  in  which  the  carbons  are  placed 
one  above  the  other  as  in  the  ordinary  arc  lamp.  The  term  lumi- 
nous arc  is  usually  applied  to  arcs  of  the  flaming  type  in  which  the 
electrodes  are  placed  one  above  the  other.  The  minor  modifications 
as  introduced  by  the  various  manufacturers  are  numerous  and  include 
such  features  as  a  magazine  supply  of  electrodes  by  which  a  new  pair 
may  be  automatically  introduced  when  one  pair  is  consumed;  feed 
and  control  mechanisms;  etc.  The  flaming  arc  presents  a  special 
problem  since  the  vapors  given  off  by  the  lamp  may  condense  on  the 
glassware  and  form  a  partially  opaque  coating,  or  they  may  interfere 
with  the  control  mechanism. 

Bremer  Arc.  The  Bremer  flaming  arc  lamp  was  introduced 
commercially  in  1899,  and  since  some  of  its  principles  are  incorporated 
in  many  of  the  lamps  on  the  market  to-day,  it  will  be  briefly  described 
here.  The  diagram  shown  in  Fig.  39  illustrates  the  main  features  of 


ELECTRIC  LIGHTING 


43 


this  lamp.  The  electrodes  are  mounted  at  an  angle  and  an  electro- 
magnet is  placed  above  the  arc  for  the  purpose  of  keeping  the  arc  from 
creeping  up  and  injuring  the  economizer,  and  also  for  the  purpose  of 
spreading  the  arc  out  and  increasing  its  surface.  The  vapor  from 
the  arc  is  condensed  on  the  economizer  and  this  coating  acts  as  a  re- 
flector, throwing  the  light  downward.  The  economizer  serves  to 
limit  the  air  supplied  to  the  arc  and  thus  increases  the  life  of  the  elec- 
trodes. The  inclined  position  of  the  carbons  was  suggested  by  the 
fact  that  in  the  impregnated  carbons  a  slag  was  formed  which  gave 
trouble  when  the  electrodes  were  mounted  in  the  usual  manner.  By 
using  the  electrodes  in 
this  position  there  is  little 
if  any  obstruction  to  the 
light  which  passes  di- 
rectly downward  from 
the  arc. 

Bremer's  original 
electrodes  contained 
compounds  of  calcium, 
strontium,  magnesium, 
etc.,  as  well  as  boracic 
acid.  Electrodes  as  em- 
ployed in  the  various 
lamps  to-day  differ 
greatly  in  their  make-up. 
Some  use  impregnated 

carbons,  others  use  carbons  with  a  core  containing  the  flaming  ma- 
terials, and  metallic  wires  are  added  in  some  cases.  The  life  of 
electrodes  for  flaming  lamps  is  not  great,  depending  upon  their  length 
and  somewhat  upon  the  type  of  lamp.  The  maximum  life  of  the 
treated  carbons  is  in  the  neighborhood  of  20  hours. 

The  color  of  the  light  from  the  flaming  arc  is  yellow  when  cal- 
cium salts  are  used  as  the  main  impregnating  compound,  and  the 
majority  of  the  lamps  installed  use  electrodes  giving  a  yellow  light. 
By  employing  more  strontium,  a  red  or  pink  light  is  produced,  while 
if  a  white  light  is  wanted,  barium  salts  are  used.  Calcium  gives  the 
most  efficient  service  and  strontium  comes  between  this  and  barium. 
The  distribution  curves  in  Fig.  40  illustrate  the  relative  economies 


Fig.  40.     Distribution  Curves  of  a  Luminous  Arc. 


44 


ELECTRIC  LIGHTING 


of  the  different  materials.  Modern  electrodes  contain  not  more  than 
15%  of  added  material  and  it  is  customary  to  find  the  salts  applied 
as  a  core  to  the  pure  carbon  sticks.  The  electrodes  are  made  of  a 
small  diameter  in  order  to  maintain  a  steady  light  and  this  partially 
accounts  for  their  short  life. 

The  feeding  mechanisms  employed  differ  greatly.  They  may  be 
classified  as:  Clock,  gravity-feed,  clutch,  motor,  and  hot-wire  mech- 
anisms. Fig.  41  illustrates  a  clock  mechanism.  This  is  a  dif- 
ferential mechanism  in  which  the 
shunt  coils  act  to  release  a  detent  / 
which  allows  the  electrodes  to  feed 
down  and  when  they  come  in  con- 
tact the  series  coils  separate  them 
to  the  proper  extent  for  maintaining 
a  suitable  arc.  In  the  gravity  feed 
an  electromagnet  is  used  to  operate 
one  carbon  in  springing  the  arc  and 
the  other  carbon  is  fed  by  gravity, 
it  being  prevented  from  dropping 
too  far  by  means  of  a  special  rib 
formed  on  the  electrode  which  comes 
in  contact  with  a  part  of  the  lamp 
structure.  Gravity  feed  is  also  em- 
ployed in  the  clutch  mechanism  but 
here  the  carbons  are  held  in  one 
position  by  an  electrically  operated 
clutch  which  releases  them  only  when 
the  current  is  sufficiently  reduced  by 
the  lengthening  of  the  arc.  In  the 

hot-wire  lamp,  the  wire  is  usually  in  series  with  the  arc;  the  contrac- 
tion and  expansion  of  this  wire  is  balanced  against  a  spring  and  the 
arc  is  regulated  by  such  contraction  or  expansion  of  the  wire.  Such 
a  lamp  is  suitable  for  either  direct  or  alternating  current.  In  the 
motor  mechanism,  as  applied  to  alternating-current  lamps,  a  metallic 
disk  is  actuated  by  differential  magnets  and  its  motion  is  transmitted 
to  the  electrodes  to  lengthen  or  shorten  the  arc  accordingly  as  the 
force  exerted  by  the  series  or  shunt  coils  predominates. 

Magnetite  Arc.     The  magnetite  arc  employs  a  copper  disk  as 


Fig.  41.    Clock  Feeding  Mechanism  for 
Luminous  Arc  Lamp. 


ELECTRIC  LIGHTING 


45 


one  electrode;  and  a  magnetite  stick — formed  by  forcing  magnetite, 
to  which  titanium  salts  are  usually  added,  into  a  thin  sheet  steel  tube — 
is  used  as  the  other  electrode.  This  lamp  gives  a  luminous  arc  of 
good  efficiency  and  the  magnetite  electrode  is  not  consumed  as  rapidly 
as  the  treated  carbons  with  the  result  that  magnetite  lamps  do  not 
require  trimming  as  frequently.  The  life  of  the  magnetite  electrode 
as  at  present  manufactured  is  from  170  to  200  hours.  A  diagram  of 
the  connections  of  this  lamp  as  manufactured  by  the  General  Electric 


Starting 
Maonet 


(s 

B 

= 

— 

f  or  ting  Resist  an  c 

1 

Fig.  42.     Diagram  of  Connections  for  Magnetite  Arc  Lamp. 

Company  is  shown  in  Fig.  42.  The  magnetite  electrode  is  placed  be- 
low. The  copper  electrode  has  just  the  proper  dimensions  to  prevent 
its  being  destroyed  by  the  arc  and  yet  it  is  not  large  enough  to  cause 
undue  condensation  of  the  arc  vapor.  Direct  current  must  be  used 
with  this  lamp,  the  current  passing  from  the  copper  to  the  magnetite. 
Table  XI  gives  some  general  data  on  the  flaming  arc,  while  Figs. 
43  and  44  give  typical  distribution  curves.  The  advantages  of  the 
flaming  arc  over  lamps  using  pure  carbon  electrodes  are:  High  effi- 
ciency; better  light  distribution;  and  better  color  of  light  for  some 


46 


ELECTRIC  LIGHTING 


purposes.  A  greater  amount  of  light  can  be  obtained  from  a  single 
unit  than  is  practical  with  the  carbon  arc.  The  disadvantages  lie 
in  the  frequent  trimming  required  and  the  expense  of  electrodes. 
Flaming  arcs  have  been  introduced  abroad,  especially  in  Germany, 
to  a  much  greater  extent  than  in  the  United  States. 

TABLE  XI 
General  Data  on  Flaming  Arcs 


VOLTS 

AMPERES 

WATTS 

MEAN   SPHERICAL 
CANDLE-POWER 

WATTS  PER  MEAN 
SPHERICAL  c.  P. 

55 

6 

330 

480 

.68 

8 

440 

800 

.55 

10 

550 

1100 

.5  - 

12 

660 

1300 

.5 

15 

825 

1700 

.49 

20 

1100 

2250 

.48 

POWER  DISTRIBUTION 

The  question  of  power  distribution  for  electric  lamps  and  other 
appliances  is  taken  up  fully  in  the  section  on  that  subject,  therefore 
it  will  be  treated  very  briefly  here.  The  systems  may  be  divided  into : 

1.  Series  distribution  systems. 

2.  Multiple-series  or  series-multiple  systems. 

3.  Multiple  or  parallel  systems. 

They  apply  to  both  alternating  and  direct  current. 

The  Series  System.  This  is  the  most  simple  of  the  three;  the 
lamps,  as  the  name  indicates,  are  connected  in  series  as  shown  in 
Fig.  45.  A  constant  load  is  necessary  if  a  constant  potential  is  to  be 
used.  If  the  load  is  variable,  a  constant-current  generator,  or  a 
special  regulating  device  is  necessary.  Such  devices  are  constant- 
current  transformers  and  constant-current  regulators  as  applied  to 
alternating-current  circuits. 

The  series  system  is  used  mostly  for  arc  and  incandescent  lamps 
when  applied  to  street  illumination.  Its  advantages  are  simplicity 
and  saving  of  copper.  Its  disadvantages  are  high  voltage,  fixed  by 
the  number  of  lamps  in  series;  the  size  of  the  machines  is  limited 
since  they  cannot  be  insulated  for  voltage  above  about  6,000;  a  single 
open  circuit  shuts  down  the  whole  system. 

Alternating-current  series  distribution  systems  are  being  used  to 
a  very  large  extent.  By  the  aid  of  special  transformers,  or  regulators, 


ELECTRIC  LIGHTING 


47 


any  number  of  circuits  can  be  run  from  one  machine  or  set  of  bus  bars, 
and  apparatus  can  be  built  for  any  voltage  and  of  any  size.  It  is  not 
customary,  however,  to  build  transformers  of  this  type  having  a  capac- 


o° 


6O°  75"  90'  75°  SO' 

Fig.  43.     Distribution  Curve  for  Flaming  Arc  Lamp. 

ity  greater  than  one  hundred  6.6-ampere  lamps  because  of  the  high 
voltage  which  would  have  to  be  induced  in  the  secondary  for  a  larger 
number  of  lamps. 

Fig.  45  gives  a  dia- 
gram of  the  connection 
of  a  single-coil  trans- 
fermer  in  service.  The 

constant-current      trans-     o°V-      IL^I^    •   JCJ4--UI      — jo° 

/o" 

20' 


Luminous  Arc    Lamp 
Direct  current   series  circuit 
Distribution 


30° 


former   most   in  use  for 
lighting  purposes  is  the    20" 
one  manufactured  by  the 
General    Electric    Com- 
pany   and   commonly 
known   as   a   tub    trans- 
former.     Fig.  46  shows  such  a  transformer  (double-coil  type)  when 
removed  from  the  case. 

Referring  to  Fig.  46,  the  fixed  coils  A  form  the  primaries  which 
are  connected  across  the  line;  the  movable  coils  B  are  the  secondaries 


40"     50'  6 0'  TO'SCrSO* 80  70*60'  50°      4-0° 

Fig.  44.     Distribution  Curve  for  a  4-Ampere, 

75-\olt,  Magnetite  Luminous  Arc  Lamp. 


48 


ELECTRIC  LIGHTING 


SECONDARY 

-X— X— X— X— X— X- 
LAMPS 


connected  to  the  lamps.  There  is  a  repulsion  of  the  coils  B  by  the 
coils  A  when  the  current  flows  in  both  circuits  and  this  force  is  bal- 
anced by  means  of  the  weights  at  W,  so  that  the  coils  B  take  a  position 
such  that  the  normal  current  will  flow  in  the  secondary.  On  light 
loads,  a  low  voltage  is  sufficient,  hence  the  secondary  coils  are  close 

together  near  the  middle  of 
the  machine  and  there  is  a 
heavy  magnetic  leakage. 
When  all  of  the  lamps  are 
on,  the  coils  take  the  posi- 
tion shown  when  the  leak- 
age is  a  minimum  and  the 
voltage  a  maximum.  When 
first  starting  up,  the  trans- 
former is  short-circuited  and 
the  secondary  coils  brought 
close  together.  The  short 
circuit  is  then  removed  and 
the  coils  take  a  position 
corresponding  to  the  load 
on  the  line. 

These  transformers  regu- 
late from  full  load  to  J  rated 
load  within  TV  ampere  of 
normal  current,  and  can  be 
run  on  short  circuit  for 

C  PRIMARY  PLUG  SWITCH  several  hours  without  over- 
heating. The  efficiency  is 
given  as  96%  for  100-light 
transformers  and  94.6%  for 
50-light  transformers  at  full 

load.  The  power  factor  of  the  system  is  from  76  to  78%  on  full 
load,  and,  owing  to  the  great  amount  of  magnetic  leakage  at  less  than 
full  load — the  effect  of  leakage  being  the  same  as  the  effect  of  an  in- 
ductance in  the  primary — the  power  factor  is  greatly  reduced,  falling 
to  62%  at  f  load,  44%  at  J  load,  and  24%  at  J  load. 

Standard  sizes  are  for  capacities  of  25-,  35-,  50-,  75-,  and  100-6.6 
ampere  enclosed  arcs,  and  they  are  also  made  for  lower  currents  in 


SHORT  CIRCUITING 
PLUG  SWITCH 


CURRENT 
TRANSFORMER 
OMIT  FOR 
25  LIGHTS 

OPEN  CIRCUITING 
PLUG  SWITCHES 


CONSTANT  CURRENT 
TRANSFORMER 


RESISTANCE 


FUSE 


WMV 

POTENTIAL 
TRANSFORMER 


PRIMARY 
BACK   VIEW 


Fig.  45. 


Wiring  Diagram  for  Single-Coil 
Transformer. 


ELECTRIC  LIGHTING 


49 


50 


ELECTRIC  LIGHTING 


the  neighborhood  of  3.3  amperes  for  incandescent  lamps.  The  low 
power  factor  of  such  a  system  on  light  loads  shows  that  a  transformer 
should  be  selected  of  such  a  capacity  that  it  will  be  fully  or  nearly 
fully  loaded  at  all  times.  The  primary  winding  can  be  constructed 
for  any  voltage  and  the  open  circuit  voltages  of  the  secondaries  are 
as  follows : 


25  light  transformer,  2,300  volts. 
35     "  "  3,200      " 

50     "  "  4,600      " 


75  light  transformer,  6,900  volts. 
100     "  "  9,200     " 


The  50-,  75-,  and  100-light  transformers  are  arranged  for  multiple 

circuit  operation,  two  circuits 
used  in  series,  and  the  vol- 
tages at  full  load  reach  4,100 
for  each  circuit  on  the  100-light 
machine. 

The  second  system,  used 
for  series  distribution  on 
alternating-current  circuits 
consists  of  a  constant-potential 
transformer,  stepping  down  the 
line  voltage  to  that  required 
for  the  total  number  of  lamps 
on  the  system,  allowing  83 
volts  for  each  lamp,  and  in 
series  with  the  lamps  is  a 
reactive  coil,  the  reactance  of 
which  is  automatically  regu- 
lated, as  the  load  is  increased 
or  decreased,  in  order  to  keep 

Fig.  47.     Current  Regulator  for  A.  C.  Series  ^ 

Distribution  Systems.  the   current   in   the  line  con- 

stant. Fig.  47  shows  such  a  regulator  and  Fig.  48  shows  this  regu- 
lator connected  in  circuit.  The  inductance  is  varied  by  the  move- 
ment of  the  coil  so  as  to  include  more  or  less  iron  in.  the  magnetic 
circuit.  Since  the  inductance  in  series  with  the  lamps  is  high  on  light 
loads,  the  power  factor  is  greatly  reduced  as  in  the  constant-current 
transformer;  and  the  circuits  should,  preferably,  be  run  fully  loaded. 
60  to  65  lamps  on  a  circuit  is  the  usual  maximum  limit. 

While  used  primarily  for  arc-light  circuits,  the  same  systems, 


ELECTRIC  LIGHTING 


51 


designed  for  lower  currents,  are  very  readily  applied  to  series  incan- 
descent systems. 

The  introduction  of  certain  flaming  or  luminous  arcs  requiring 
direct  current  for  their  operation  has  led  to  the  use  of  the  mercury  arc 
rectifier  in  connection  with  series  circuits  on  alternating-current 
systems.  A  constant-current  transformer  is  used  to  regulate  for  the 
proper  constant  current  in  its  second- 
ary winding,  and  this  secondary  current 
is  rectified  by  means  of  the  mercury  arc 
rectifier  for  the  lamp  circuit.  In  the 
recent  outfits  the  rectifier  tubes  are 
immersed  in  oil  for  cooling.  While 
this  rectifier  was  first  introduced  for 
the  operation  of  luminous  arc  lamps, 
there  is  no  reason  why  it  should  not 
be  used  with  any  series  lamp  requiring  KICKING 
direct  current,  provided  the  system  is 
designed  for  the  current  taken  by  such 
lamps.  With  this  system  any  commer- 
cial frequency  may  be  used.  Sets  are 
constructed  for  25-,  50-,  and  75-light 
circuits.  They  have  a  combined  effi- 
ciency, transformer  and  rectifier  tube, 
of  85%  to  90%,  and  operate  at  a  power 
factor  of  from  65%  to  70%.  Fig.  49 
gives  a  diagram  of  the  circuit  and 

rectifier  connections  USed  with  a  single-  Fig.  48.    Wiring  Diagram  Showing  In- 
,  n ,  troduction  of  the  Current  Regulator. 

tube  outfit. 

Multiple= Series  or  Series=  Multiple  Systems.  These  combine 
several  lamps  in  series,  and  these  series  groups  in  multiple,  or  several 
lamps  in  multiple  and  these  multiple  groups  in  series,  respectively. 
They  have  but  a  limited  application. 

Multiple  or  Parallel  Systems  of  Distribution.  By  far  the  largest 
number  of  lamps  in  service  are  connected  to  parallel  systems  of  dis- 
tribution. In  this  system,  the  units  are  connected  across  the  lines 
leading  to  the  bus  bars  at  the  station,  or  to  the  secondaries  of  con- 
stant-potential transformers.  Fig.  50  shows  a  diagram  of  ten  lamps 
connected  in  parallel.  The  current  delivered  by  the  machine  de- 


C.P.  STEP  UP 
OR  STtlPDOWN 
TRANSFORMER 


UGHTNING 
ARRESTERS 


52 


ELECTRIC  LIGHTING 


pends  directly  on  the  number  of  lamps  connected  in  service,  the  vol- 
tage of  the  system  being  kept  constant. 

Inasmuch  as  the  flow  of  current  in  a  conductor  is  always  accom- 
. —  panied  by  a  fall 

-tzsACBusses\ 


Fig.  49. 

various 
systems 

1. 

2. 
3. 
4. 


of  potential  equal 
to  the  product  of 
the  current  flow- 
ing into  the  resist- 
ance of  the  con- 
ductor, the  lamps 
at  the  end  of  the 
system  shown 
will  not  have  as 
high  a  voltage 
impressed  upon 
them  as  those 
nearer  the  ma- 
chine.  This 
drop  in  potential 
is  the  most  seri- 
ous obstacle  that 
we  have  to  over- 

AViring  Diagram  for  A.  C.  System  Showing  Introduc-     come  in  multiple 
tion  of  Mercury  Arc  Rectifier. 

systems,     and 

schemes  have  been  adopted  to  aid  in  this  regulation.     The 
may  be  classified  as : 

Cylindrical  conductors,  parallel  feeding. 

Conical 

Cylindrical  anti-parallel  feeding. 

Conical 


In  the  cylindrical  conductor,  parallel-feeding  system,  the  con- 
ductors, A,  B,C,  D,  Fig.  50,  are  of  the  same  size  throughout  and  are 
fed  at  the  same  end  by  the  generator.  The  voltage  is  a  minimum 
at  the  lamps  E  and  a  maximum  at  the  lamps  F;  the  value  of  the 
voltage  at  any  lamp  being  readily  calculated. 

By  a  conical  or  tapering  conductor  is  meant  a  conductor  whose 
diameter  is  so  proportioned  throughout  its  length  that  the  current, 
divided  by  the  cross-section,  or  the  current  density,  is  a  constant 


ELECTRIC  LIGHTING 


53 


Fig.  50.     Parallel  Feeding  System. 


Fig.  51.     Anti-parallel  Feeding  System. 


quantity.     Such  a  conductor  is  approximated  in  practice  by  using 
smaller  sizes  of  wire  as  the  current  in  the  lines  becomes  less. 

In  an  anti-parallel  system,  the  current  is  fed  to  the  lamps  from 
opposite  ends  of  the  system,  as  shown  in  Fig.  51. 

Multiple=Wire  Systems.  In  order  to  take  advantage  of  a  higher 
voltage  for  distribution  of  power  to  the  lighting  circuits,  three-  and 
five-wire  systems  have  been  introduced,  the  three-wire  system  being 
used  to  a  very  large  extent.  In  this  system,  three  conductors  are 
used,  the  voltage  from  each  A  B 

outside  conductor  to  the 
middle  neutral  conductor 
being  the  same  as  for  a 
simple  parallel  system.  Fig. 
52  gives  a  diagram  of  this. 
By  this  system  the  amount 
of  copper  required  for  a  giv- 
en number  of  lamps  is  from 
five-sixteenths  to  three- 
eighths  of  the  amount 
required  for  a  two-wire  dis- 
tribution, depending  on  the 

size  of  the  neutral  con-  Fig  52     Three.wire  System. 

ductor.     The   saving  of 

copper  together  with  the  disadvantages  of  the  system  is  more  fully 
treated  in  the  paper  on  "Power  Transmission." 

ILLUMINATION 

Illumination  may  be  defined  as  the  quality  and  quantity  of  light 
which  aids  in  the  discrimination  of  outline  and  the  perception  of 
color.  Not  only  the  quantity,  but  the  quality  of  the  light,  as  well  as 
the  arrangement  of  the  units,  must  be  considered  in  a  complete  study 
of  the  subject  of  illumination. 

Unit  of  Illumination.  The  unit  of  illumination  is  the  joot- 
candle  and  its  value  is  the  amount  of  light  falling  on  a  surface  at  a 
distance  of  one  foot  from  a  source  of  light  one  candle-power  in  value. 
The  law  of  inverse  squares — namely,  that  the  illumination  from  a 
given  source  varies  inversely  as  the  square  of  the  distance  from  the 
source — shows  that  the  illumination  at  a  distance  of  two  feet  from  a 


54 


ELECTRIC  LIGHTING 


single    candle-power    unit    is   .25    foot-candles.      For    further    con- 
sideration of  the  law  of  inverse  squares,  see  "Photometry." 

Illumination  may  be  classified  as  use/lit — when  used  for  the 
ordinary  purposes  of  furnishing  light  for  carrying  on  work,  taking 
the  place  of  daylight;  and  scenic — when  used  for  decorative  lighting 
such  as  stage  lighting,  etc.  The  two  divisions  are  not,  as  a  rule, 
distinct,  but  the  one  is  combined  with  the  other. 

Intrinsic  Brightness.  By  intrinsic  brightness  is  meant  the 
amount  of  light  emitted  per  uirt  surface  of  the  light  source.  Table 
XII  gives  the  intrinsic  brightness  of  several  light  sources. 

TABLE  XII 
Intrinsic  Brilliancies  in  Candle-Power  per  Square  Inch 


SOURCE 

BRILLIANCY 

NOTES 

Sun  in  zenith 

600,000  ) 

Sun  at  30  degrees  elv. 

500,000  £ 

Rough  equivalent  values,   tak- 

Sun on  horizon 

2,000  ) 

ing  account  of  absorption 

10,000) 

Arc  light 

to     [• 

Maximum     about     200,000     in 

100,000  ) 

crater 

Calcium  light 

5,000 

Nernst  "glower" 

1,000 

Unshaded 

Incandescent  lamp 

200-300 

Depending  on  efficiency 

Enclosed  arc 

75-100 

Opalescent  inner  globe 

Acetylene  flame 

75-100 

Welsbach  light 

20  to  25 

Kerosene  light 

4  to    8 

Variable 

Candle 

3  to    4 

Gas  flame 

3  to    8 

Variable 

Incandescent  (frosted) 

2  to    5 

Opal  shaded  lamps,  etc. 

0.5  to    2 

Regular  Reflection.  Regular  reflection  is  the  term  applied  to 
reflection  of  light  when  the  reflected  rays  are  parallel.  It  is  of  such 
a  nature  that  the  image  of  the  light  source  is  seen  in  the  reflection. 
The  reflection  from  a  plane  mirror  is  an  example  of  this.  It  is  useful 
in  lighting  in  that  the  direction  of  light  may  be  changed  without  com- 
plicating calculations  aside  from  deductions  necessary  to  compensate 
for  the  small  amount  of  light  absorbed. 

Irregular  Reflection.  Irregular  reflection,  or  diffusion,  consists 
of  reflection  in  which  the  reflected  rays  of  light  are  not  parallel  but 
take  various  directions,  thus  destroying  the  image  of  the  light  source. 
Rough,  unpolished  surfaces  give  such  reflection.  Smooth,  unpolished 
surfaces  generally  give  a  combination  of  two  kinds  of  reflection. 


ELECTRIC  LIGHTING  55 

Diffused  reflection  is  very  important  in  the  study  of  illumination 
inasmuch  as  diffused  light  plays  an  important  part  in  the  lighting  of 
interiors.  This  form  of  reflection  is  seen  in  many  photometer  screens. 
Light  is  also  diffused  when  passing  through  semi-transparent  shades 
or  screens. 

In  considering  reflected  light,  we  find  that,  if  the  surface  on 
which  the  light  falls  is  colored,  the  reflected  light  may  be  changed  in 
its  nature  by  the  absorption  of  some  of  the  colors.  Since,  as  has  been 
said,  in  interior  lighting  the  reflected  light  forms  a  large  part  of  the 
source  of  illumination,  this  illumination  will  depend  upon  the  nature 
and  the  color  of  the  reflecting  surfaces. 

Whenever  light  is  reflected  from  a  surface,  either  by  direct  or 
diffused  reflection,  a  certain  amount  of  light  is  absorbed  by  the  surface. 
Table  XIII  gives  the  amount  of  white  light  reflected  from  different 
materials. 

TABLE  XIII 
Relative  Reflecting  Power 


MATERIAL 


White  blotting  paper 

White  cartridge  paper 

Chrome  yellow  paper 

Orange  paper 

Yellow  wall  paper 

Light  pink  paper 

Yellow  cardboard 

Light  blue  cardboard 

Emerald  green  paper 

Dark  brown  paper 

Vermilion  paper 

Blue-green  paper 

Black  paper 

Black  cloth 

Black  velvet 


82 
80 
62 
50 
40 
36 
30 
25 
18 
13 
12 
12 
5 


1.2 


From  this  table  it  is  seen  that  the  light-colored  papers  reflect  the 
light  well,  but  of  the  darker  colors  only  yellow  has  a  comparatively 
high  coefficient  of  reflection.  Black  velvet  has  the  lowest  value,  but 
this  only  holds  when  the  material  is  free  from  dust.  Rooms  with 
dark  walls  require  a  greater  amount  of  illuminating  power,  as  will  be 
seen  later. 

Useful  illumination  may  be  considered  under  the  following 
heads: 


56  ELECTRIC  LIGHTING 

1.  Residence  Lighting. 

2.  Lighting  of  Public  Halls,  Offices,  Drafting  Rooms,  Shops,  etc. 

3.  Street  Lighting. 

RESIDENCE  LIGHTING 

Type  of  Lamps.  The  lamps  used  for  this  class  of  lighting  are 
limited  to  the  less  powerful  units  —  namely,  incandescent  or  Nernst 
lamps  varying  in  candle-power  from  8  to  50  per  unit.  These  should 
always  be  shaded  so  as  to  keep  the  intrinsic  brightness  low.  The 
intrinsic  brilliancy  should  seldom  exceed  2  to  3  candle-power  per 
square  inch,  and  its  reduction  is  usually  accomplished  by  appropriate 
shading.  Arc  lights  are  so  powerful  as  to  be  uneconomical  for 
small  rooms,  while  the  color  of  the  mercury-vapor  light  is  an  additional 
objection  to  its  use. 

Plan  of  Illumination.  Lamps  may  be  selected  and  so  located 
as  to  give  a  brilliant  and  fairly  uniform  illumination  in  a  room;  but  this 
is  an  uneconomical  scheme,  and  the  one  more  commonly  employed 
is  to  furnish  a  uniform,  though  comparatively  weak,  ground  illumi- 
nation, and  to  reinforce  this  at  points  where  it  is  necessary  or  desirable. 
The  latter  plan  is  satisfactory  in  almost  all  cases  and  the  more  eco- 
nomical of  the  two. 

While  the  use  of  units  of  different  power  is  to  be  recommended, 
where  desirable,  lights  differing  in  color  should  not  be  used  for  lighting 
the  same  room.  As  an  exaggerated  case,  the  use  of  arc  with  incan- 
descent lamps  might  be  mentioned.  The  arcs  being  so  much  whiter 
than  the  incandescent  lamps,  the  latter  appear  distinctly  yellow  when 
the  two  are  viewed  at  the  same  time. 

Calculation  of  Illumination.  Jn  determining  the  value  of  illumi- 
nation, not  only  the  candle-power  of  the  units,  but  the  amount  of  re- 
flected light  must  be  considered  for  the  given  location  of  the  lamps. 
Following  is  a  formula  based  on  the  coefficient  of  reflection  of  the 
walls  of  the  room,  which  serves  for  preliminary  calculations: 


d* 

I  =  Illumination  in  foot-candles. 
c.p.  =  Candle-power  of  the  unit. 
Jc  =  Coefficient  of  reflection  of  the  walls. 
d  =  distance  from  the  unit  in  feet. 


ELECTRIC  LIGHTING  57 

Where  several   units  of  the  same  candle-power  are  used  this 
formula  becomes: 

>   (—        —        —  1 

d2         d2t        d2^  I  -  k 

c.p. j p  — 

( -77-  4~  —79    +  ~~TT  ~f~ 


1-  k 

where  d,  dv  dv  etc.,  equal  the  distances  from  the  point  considered  to 
the  various  light  sources.  If  the  lamps  are  of  different  candle-power, 
the  illumination,  may  be  determined  by  combining  the  illumination 
from  each  source  as  calculated  separately.  An  example  of  calculation 
is  given  under  "Arrangement  of  Lamps." 

The  above  method  is  not  strictly  accurate  because  it  does  not 
take  account  of  the  angle  at  which  the  light  from  each  one  of  the 
sources  strikes  the  assumed  plane  of  illumination.  If  the  ray  of 

C  T) 

light  is  perpendicular  to  the  plane,  the  formula  I  =  —^  gives  cor- 
rect values.  If  a  is  the  angle  which  the  ray  of  light  makes  with  a  line 
drawn  from  the  light  source  perpendicular  to  the  assumed  plane, 

then   the  formula  becomes  I  =    C+  X  %***  '•    Therefore,  by 

or 

multiplying  the  candle-power  value  of  each  light  source  in  the  direc- 
tion of  the  illuminated  point  by  the  cosine  of  each  angle  a,  a  more 
accurate  result  will  be  obtained. 

It  is  readily  seen  that  the  effect  of  reflected  light  from  the  ceilings 
is  of  more  importance  than  that  from  the  floor  of  a  room.  The  value 
of  k,  in  the  above  formula,  will  vary  from  60%  to  10%,  but  for  rooms 
with  a  fairly  light  finish  50%  may  be  taken  as  a  good  average  value. 

The  amount  of  illumination  will  depend  on  the  use  to  be  made  of 
the  room.  One  foot-candle  gives  sufficient  illumination  for  easy 
reading,  when  measured  normal  to  the  page,  and  probably  an  illumi- 
nation of  .5  foot-candle  on  a  plane  3  feet  from  the  floor  forms  a  suffi- 
cient ground  illumination.  The  illumination  from  sunlight  reflected 
from  white  clouds  is  from  20  foot-candles  up,  while  that  due  to  moon- 
light is  in  the  neighborhood  of  .03  foot-candles.  It  is  not  possible  to 
produce  artificially  a  light  equivalent  to  daylight  on  account  of  the 


58  ELECTRIC  LIGHTING 

great  amount  of  energy  that  would  be  required  and  the  difficulty  of 
obtaining  proper  diffusion. 

The  method  of  calculating  the  illumination  of  a  room  that  has 
just  been  described  is  known  as  the  point-by-point  method  and  it 
gives  very  accurate  results  if  account  is  taken  of  the  angle  at  which 
the  light  from  each  source  strikes  the  plane  of  illumination  and  if 
the  light  distribution  curves  of  the  units,  and  the  value  of  k,  have  been 
carefully  determined.  Under  these  conditions  the  calculations  be- 
come extended  and  complicated  and  methods  only  approximate,  but 
simpler  in  their  application,  are  being  introduced.  One  method, 
which  gives  good  results  when  applied  to  fairly  large  interiors,  makes 
the  flux  of  light  from  the  light  sources  the  basis  of  calculation  of  the 
average  illumination. 

Flux  of  light  is  measured  in  lumens  and  a  lumen  may  be  defined 
as  the  amount  of  light  which  must  fall  on  one  square  foot  of  surface 
in  order  to  produce  a  uniform  illumination  of  an  intensity  of  one  foot- 
candle.  A  source  of  light  giving  one  candle-power  in  every  direction 
and  placed  at  the  center  of  a  sphere  of  one  foot  radius  would  give  an 
illumination  of  one  foot-candle  at  every  point  in  the  surface  of  the 
sphere  and  the  total  flux  of  light  would  be  4?r,  or  12.57,  lumens  since 
the  area  of  the  sphere  would  be  4?r,  or  12.57,  sq.  ft.  A  lamp  giving 
one  mean  spherical  candle-power  gives  a  flux  of  12.57  lumens  and 
the  total  flux  of  light  from  any  source  is  obtained  by  multiplying  its 
mean  spherical  candle-power  by  12.57.  In  calculating  illumination 
it  is  customary  to  determine  the  illumination  on  a  plane  about  30 
inches  from  the  floor  for  desk  work,  and  about  42  inches  from  the 
floor  for  the  display  of  goods  on  counters.  If  we  determine  the  total 
number  of  lumens  falling  on  this  plane  and  divide  this  number  by 
the  area  of  the  plane,  we  obtain  the  average  illumination  in  foot- 
candles.  This  of  course  tells  us  nothing  about  the  maximum  or 
minimum  value  of  the  illumination  and  such  values  must  be  obtained 
by  other  methods  if  they  are  desired.  Reflected  light,  other  than  that 
covered  by  the  distribution  curve  of  the  light  unit  including  its  re- 
flector, is  usually  neglected  in  this  method  of  calculation. 

We  may  assume  that  in  large  rooms  the  light  coming  from  the 
lamp  within  an  angle  of  75  degrees  from  the  vertical  reaches  the  plane 
of  illumination.  In  smaller  rooms  this  angle  should  be  reduced  to 
about  60  degrees.  In  order  to  determine  the  flux  of  light  within  this 


ELECTRIC  LIGHTING  59 

angle  a  Rousseau  diagram,  which  is  described  later,  should  be  drawn. 
By  the  means  of  this  diagram  the  average  candle-power  of  the  light 
source  within  the  angle  assumed  may  be  readily  determined  and  this 
mean  value,  multiplied  by  12.57,  will  give  the  flux  of  light  in  lumens. 
This  method  of  calculation,  together  with  some  guides  for  its  rapid 
application,  is  described  by  Messrs.  Cravath  and  Lansingh  in  the 
"Transactions  of  the  Illuminating  Engineering  Society,  1908."  The 
same  authorities  give  the  following  useful  data: 

To  determine  the  watts  required  per  square  foot  of  floor  area, 
multiply  the  intensity  of  illumination  desired  by  the  constants  given 
as  follows: 

INTENSITY  CONSTANTS  FOR  INCANDESCENT  LAMPS 

Tungsten  lamps  rated  at  1.25  watts  per  horizontal  candle-power;  clear 
prismatic  reflectors,  either  bowl  or  concentrating;  large  room;  light 
ceiling;  dark  walls;  lamps  pendant;  height  from  8  to  15  feet  .25 

Same  with  very  light  walls 20 

Tungsten  lamps  rated  at  1.25  watts  per  horizontal  candle-power;  pris- 
matic bowl  reflectors  enameled;  large  room;  light  ceiling;  dark 

walls;  lamps  pendant,   height  from   8   to    15  feet .29 

Same  with  very  light  walls 23 

Gem  lamps  rated  at  2.5  watts  per  horizontal  candle-power;  clear  pris- 
matic reflectors  either  concentrating  or  bowl;  large  room;  light 

ceiling;  dark  walls;  lamps  pendant;  height  from  8  to  15  feet 55 

Same  with  very  light  walls 45 

Carbon  filament  lamps  rated  at  3.1  watts  per  horizontal  candle-power; 
clear  prismatic  reflectors  either  bowl  or  concentrating;  light  ceiling; 
dark  walls;  large  room;  lamps  pendant;  height  from  8  to  15  feet. .  .65 

Same  with  very  light  walls 55 

Bare  carbon  filament  lamps  rated  at  3.1  watts  per  horizontal  candle- 
power;  no  reflectors;  large  room;  very  light  ceiling  and  walls; 

height  from  10  to  14  feet .75  to  1.5 

Same;  small  room;  medium  walls 1 .25  to  2.0 

Carbon  filament  lamps  rated  at  3.1  watts  per  horizontal  candle-power; 
opal  dome  or  opal  cone  reflectors;  light  ceiling;  dark  walls;  large 

room;  lamps  pendant;  height  from  8  to  15  feet 70 

Same  with  light  walls 60 

INTENSITY  CONSTANTS  FOR  ARC  LAMPS 

5-ampere,  enclosed,  direct-current  arc  on  110-volt  circuit;  clear  inner, 
opal  outer  globe;  no  reflector;  large  room;  light  ceiling;  medium 
walls;  height  from  9  to  14  feet 50 

Arrangement  of  Lamps.  An  arrangement  of  lamps  giving  a 
uniform  illumination  cannot  be  well  applied  to  residences  on  account 
of  the  number  of  units  required,  and  the  inartistic  effect.  We  are 


60 


ELECTRIC  LIGHTING 


limited  to  chandeliers,  side  lights,  or  ceiling  lights,  in  the  majority 
of  cases,  with  table  or  reading  lamps  for  special  illumination. 

When  ceiling  lamps  are  used  and  the  ceilings  are  high,  some 
form  of  reflector  or  reflector  lamp  is  to  be  recommended.     In  any 
case  where  the  coefficient  of  reflection  of  the 
I  ceilings  is  less  than  40%,  it  is  more  economical 
to  use  reflectors.     When  lamps   are   mounted 
on    chandeliers,  the  illumination  is    far   from 
uniform,  being  a  maximum  in  the  neighbor- 
hood of  the  chandelier  and  a  minimum  at  the 
corners  of  the  room.     By  combining  chande- 
liers with  side  lights  it  is  generally  possible  to 
I  ro  §e^  a  satisfactory  arrangement  of  lighting  for 
J  small   or   medium-sized  rooms. 
<v|  As  a  check  on  the  candle-power  in  lamps 

I  .  .  require^  we  have  the  following : 

I  *^ 

*•  *—————'  T  For  brilliant    illumination    allow    one    candle- 


6'- 


Fig.  53.   Diagram  Showing   Power  Per  two  square  feet  of  floor  space.     In  some 
Method  of  Calculating      particular  cases,  such  as  ball  rooms,  this  may  be 
Room  Illumination.        increased  to  one  candle-power  per  square  foot. 

For  general  illumination  allow  one  candle-power 

for  four  square  feet  of  floor  space,  and  strengthen  this  illumination  with  the 
aid  of  special  lamps  as  required.  The  location  of  lamps  and  the  height  of 
ceilings  will  modify  these  figures  to  some  extent. 

As  an  example  of  the  calculation  of  the  illu-  y 
mination  of  a  room  with  different  arrangements 
of  the  units  of  light,  assume  a  room  16  feet    | 
square,  12  feet  high,  and  with  walls  having  a  S 
coefficient  of  reflection  of  50%.     Consider  first 
the  illumination  on  a  plane  3  feet  above  the 
floor  when  lighted  by  a  single  group  of  lights 
mounted  at  the  center  of  the  room  3  feet  below 
the  ceiling.     If  a  minimum  value  of  .5  foot- 
candle  is  required  at  the  corner  of  the  room, 
we  have  the  equation  (first  method  outlined) : 

.5  =  c.  v. — ^^ — X 


12.82 


1  -  .5 


Since  d  =i/82  +  82  +  62 
Fig.  53) 


=  12.8  (see 


f  ^mmsf°0rnFour 
'side  Vail. 


ELECTRIC  LIGHTING 


Solving  the  above  for  the  value  of  c.  p.,  we  have 

c.  p.  =-  -  '5  .5  X  82  ==  41 

^ 


.5 

Three  16-candle-power  lamps  would  serve  this  purpose  very 
well. 

Determining  the  illumination  directly  under  the  lamp,  we  have: 

I  =  48  X  ~  X  -—  ^r-  =  |J  X  2  = 
62  1  -  .5         36 

2.7  foot-candles,  or  five  times  the  value  of  the  illumination  at  the 
corners  of  the  room. 

Next  consider  four  8-candle-power  lamps  located  on  the  side 
walls  8  feet  above  the  floor,  as  shown  in  Fig.  54.  Calculating  the 
illumination  at  the  center  of  the  room  on  a  plane  three  feet  above 
the  floor,  we  have: 


T 


N 

1  -  . 


89        89        89        89        1  -  .5 
ft  =  82  +  52  =  64   +   25    -  89 

I  =  8  X  -£-  X  2  =  .72  foot-candles 

Ot/ 

The  illumination  at  the  corner  of  the  room  would  be: 

\  on  on  o  A  e          o/ic    ' 


89        89       345      345  '     1  -  .5 
)  X  2  =  .45  foot-candles. 


In  a  similar  manner  the  illumination  may  be  calculated  for  any 
point  in  the  room,  or  a  series  of  points  may  be  taken  and  curves  plotted 
showing  the  distribution  of  the  light,  as  well  as  the  areas  having  the 
same  illumination.  Where  refined  calculations  are  desired,  the  dis- 
tribution curve  of  the  lamp  must  be  used  for  determining  the  candle- 
power  in  different  directions.  Fig.  55  shows  illumination  curves  for 
the  Meridian  lamp  as  manufactured  by  the  General  Electric  Com- 
pany. This  is  a  form  of  reflector  lamp  made  in  two  sizes,  25  or  50 
candle-power.  Fig.  56  gives  the  distribution  curves  for  the  50- 
candle-power  unit.  Similar  incandescent  lamps  are  now  being 
manufactured  by  other  companies. 


ELECTRIC  LIGHTING 


Table  XIV  gives  desirable  data  in  connection  with  the  u^  of 
the  Meridian  lamp. 


Fig.  55.     Illumination  Curves  for  a  G.  E.  Meridian  Lamp. 

TABLE  XIV 
Illuminating  Data  for  Meridian  Lamps 


No.  1  Lamp  (60  Watts)  No  2  Lamp(120  Watts) 

Class  Service 

Light 
Intensity 
in  Foot- 
candles 

Height  of 
Lamp  and 
Diameter 
of  Uni- 
formly 
Lighted 

Distance 
between 
Lamps 
when  Two 
or  more 
are  Used 

Height  of 
Lamp  and 
Diameter 
of  Uni- 
formly 
Lighted 

Distance 
between 
Lamps 
when  Two 
or  more 
are  Used 

per  Sq.  Ft. 
of  Area 
Lighted 
with 
either 
Lamp 

Desk  or  Reading 

Tnhlp 

3 
2 

2.9  feet 
3.5 

4  .  9  feet 
6 

4       feet 
5 

7      feet 

8.5     ' 

2.50 
1.66 

1? 

4 

7       " 

5.75  ' 

9.8     ' 

1.25 

1 

5 

8.5    " 

7 

12 

0.83 

General  Lighting 

1 

5.75 

9.8    " 

8.2    ' 

13.9      ' 

0.62 

I 

7 

12 

10 

11 

0.41 

By  means  of  the  Weber,  or  some  other  form  of  portable  photom- 
ete*,  carves  as  plotted  from  calculations  may  be  readily  checked 
after  the  lamps  are  installed.  When  lamps  are  to  be  permanently 
located,  the  question  of  illumination  becomes  an  important  one,  and 
it  may  be  desirable  to  determine,  by  calculation,  the  illumination 
curves  for  each  room  before  installing  the  lamps.  This  applies  to 
the  lighting  of  large  interiors  more  particularly  than  to  residence 
lighting.  The  point-by-point  method  of  calculation  is  used  for 


ELECTRIC  LIGHTING 


63 


very  accurate  work  when  the  system  of  illumination  admits  of  this 
method.  Other  methods  are  often  simpler  and  sufficiently  accurate 
for  practical  work. 


no 


30°  20°  10°  0°  10°  20°  30° 

Fig.  56.     Distribution  Curve  for  a  G.  E.  50-c.  p.  Meridian  Lamp. 

Dr.  Louis  Bell  gives  the  following  in  connection  with  residence 

lighting: 

TABLE  XV 

Residence  Lighting  Data 


ROOM 

8 

C.  P. 

16 

C.  P. 

32 

C.P. 

SQ.  FT. 

PER  C.P. 

REMARKS 

Hall,  15'  X  20'  
Library  20'  X  20' 

8 
12 

1 

4.7 

*5      1 

8-c  p   reflector  lamps 

Reception  room,  15'  X  15'  .. 
Music  room,  20'  X  25'  
Dining  room,  15'  X  20'  
Billiard  room,  15'  X  20'  
porch  

4 
12 
14 

2 

4 
1 

7.0 
3.0 
2.7 
2.3 

8  reflector  lamps 
32-c.p.with  reflectors 

Bedrooms  (6),  15'  X  15'  
Dressing  rooms  (2),  10'  X  15'. 
Servants'  rooms  (3),  10'  X15' 
Bathrooms  (3),  8'  X  10'  
Kitchen,  15'  X  15'  ) 

14 
4 
3 
3 

3 

7.0 
4.7 
9.4 
5.0 

Pantry,  10'  X  15'   f  ' 
Halls  ) 

10 

3 

Cellar  f  
Closets  (4)  

4 

Reflector  lamps 

Total  

64 

30 

8 

^64  ELECTRIC  LIGHTING 

LIGHTING  OF  PUBLIC  HALLS,  OFFICES,  ETC. 

Lighting  of  public  halls  and  other  large  interiors  differs  from  the 
illumination  of  residences  in  that  there  is  usually  less  reflected  light, 
and,  again,  the  distance  of  the  light  sources  from  the  plane  of  illumi- 
nation is  generally  greater  if  an  artistic  arrangement  of  the  lights  is 
to  be  brought  about.  This  in  turn  reduces  the  direct  illumination. 
The  primary  object  is,  however,  as  in  residence  lighting,  to  produce 
a  fairly  uniform  ground  illumination  and  to  superimpose  a  stronger 
illumination  where  necessary.  An  illumination  of  .5  foot-candle  for 
the  ground  illumination  may  be  taken  as  a  minimum. 

In  the  lighting  of  large  rooms  it  is  permissible  to  use  larger  light 
units,  such  as  arc  lamps  and  high  candle-power  Nernst  or  incan- 
descent units,  while  for  factory  lighting  and  drafting  rooms,  where 
the  color  of  the  light  is  not  so  essential,  the  Cooper-Hewitt  lamp  is 
being  introduced.  High  candle-power  reflector  lamps,  such  as  the 
tungsten  lamp,  are  being  used  to  a  large  extent  for  offices  and  drafting 
rooms. 

The  choice  of  the  type  of  lamp  depends  on  the  nature  of  the 
work.  Where  the  light  must  be  steady,  incandescent  or  Nernst 
lamps  are  to  be  preferred  to  the  arc  or  vapor  lamps,  though  the  latter 
are  often  the  more  efficient.  When  arcs  are  used,  they  must  be  care- 
fully shaded  so  as  to  diffuse  the  light,  doing  away  with  the  strong 
shadows  due  to  portions  of  the  lamp  mechanism,  and  to  reduce  the 
intrinsic  brightness.  Such  shading  will  be  taken  up  under  the  head- 
ing " Shades  and  Reflectors."  Arcs  are  sometimes  preferable  to 
incandescent  lamps  when  colored  objects  are  to  be  illuminated,  as  in 
stores  and  display  windows. 

In  locating  lamps  for  this  class  of  lighting,  much  depends  on  the 
nature  of  the  building  and  on  the  degree  of  economy  to  be  observed. 
For  preliminary  determination  of  the  location  of  groups,  or  the  illumi- 
nation when  certain  arrangement  of  the  units  is  assumed,  the  prin- 
ciples outlined  under  "Residence  Lighting"  may  be  applied.  It  has 
been  found  that  actual  measurements  show  results  approximating 
closely  such  calculated  values. 

When  arcs  are  used  they  should  be  placed  fairly  high,  twenty 
to  twenty-five  feet  when  used  for  general  illumination  and  the  ceilings 
are  high.  They  should  be  supplied  with  reflectors  so  as  to  utilize 
the  light  ordinarily  thrown  upwards.  When  used  for  drafting-room 


ELECTRIC  LIGHTING 


65 


work,  they  should  be  suspended  from  twelve  to  fifteen  feet  above 
the  floor,  and  special  care  must  be  taken  to  diffuse  the  light. 

Incandescent  lamps  may  be  arranged  in  groups,  either  as  side 
lights  or  mounted  on  chandeliers,  or  they  may  be  arranged  as  a  frieze 
running  around  the  room  a  few  feet  below  the  ceiling.  The  last 
named  arrangement  of  lights  is  one  that  may  be  made  artistic,  but  it 
is  uneconomical  and  when  used  should  serve  for  the  ground  illumina- 
tion only.  Reflector  lights  may  be  used  for  this  style  of  work  and 
the  lights  may  be  entirely  concealed  from  view,  the  reflecting  prop- 
erty of  the  walls  being  utilized  for  distributing  the  light  where  needed. 

Ceiling  lights  should  preferably  be  supplied  with  reflectors, 
especially  when  the  ceilings  are  high. 

Indirect  lighting  is  employed  to  some  extent.  By  indirect 
lighting  \ve  mean  a  system  af  illumination  in  which  the  light  sources 
are  concealed  and  the  light  from  them  is  reflected  to  the  room  by  the 
walls,  or  ceilings,  or  other  surfaces;  or  in  which  the  light  sources  are 
placed  above  a  diffusing  panel.  In  the  latter  case  the  diffusing  plate 
appears  to  be  the  source  of  light.  In  some  cases  the  walls  themselves 
are  shaped  and  constructed  so  as  to  form  the  reflectors  for  the  light 
units  (cove  lighting),  but  in  others  all  of  the  reflecting  surfaces,  except 
the  side  walls  and  ceiling,  are  made  portions  of  the  lamp  fixtures. 

Tables  XVI  and  XVII  give  data  on  arc  and  mercury-vapor 
lamps  for  lighting  large  rooms.  Table  XVII  refers  to  arc  lights  as 
aerially  installed. 

TABLE  XVI 

Cooper=Hewitt  Lamps 


SERVICE 

HEIGHT  OF  LAMP 

C.  P.  OF  UNIT 

Av.  AREA  PER  LAMP 
IN  SQUARE  FEET 

Foundry 

10-15  ft. 

300 

900 

« 

20-25 

700 

2250 

Machine  shop 

10-15 

300 

500 

Erecting  shop 
Drafting  room 

20-30 
15 

700 
300 

1250 
300 

(i           « 

20 

700 

400 

Offices 

10-15 

300 

400 

« 

20-25 

700 

750 

<  Vdinary  labor 

10-15 

300 

1100 

«    '       a 

20-25 

700 

2750 

66 


ELECTRIC  LIGHTING 


K   a 
pj   « 

W    0 


I: 


8 

00  CO 


8        3§8 


o*  £ 


11 

P 


o       10 

-O<N       ^H 


£ 

E 

->1 

Si 

-3     d 
03     ^ 

JS  ° 

bJC 

C 


8 


O01  CO 


.-H  ^HCOCO 


3    .      ^ 

|^^  :§° 
3     ^  : 


<N 

O    CO 


Q 

fi  : 


,o5 
{H  1-1 


S 


OTH 

TO 


<N       .   O 


a  : 


rt      •     c3      • 

«  :  -8   : 

a  -a  • 


:».*  i 


ELECTRIC  LIGHTING  67 

Measurements  taken  in  well-lighted  rooms  having  a  floor  space 
of  from  1,000  to  5,000  square  feet  show  an  average  of  3  to  3.5  square 
feet  per  candle-power.  About  2.5  square  feet  per  candle-power 
should  be  allowed  when  brilliant  lighting  is  required  or  the  ceilings 
are  very  high,  while  3.75  square  feet  per  candle-power  will  give  good 
illumination  when  lights  are  well  distributed  and  there  is  considerable 
reflected  light. 

In  factory  and  drafting  room  lighting,  the  lamps  must  be  arranged 
to  give  a  strong  light  where  most  needed,  and  located  to  prevent  such 
shadows  as  would  interfere  with  the  work. 

STREET  LIGHTING 

In  studying  the  lighting  of  streets  and  parks,  we  find  that,  except 
in  special  cases,  such  as  narrow  streets  and  high  buildings,  there  is 
no  reflected  light  which  aids  the  illumination  aside  from  that  due  to 
special  shades  or  reflectors  on  the  lamp  itself.  Such  reflectors  are 
necessary  if  the  light  ordinarily  thrown  above  the  horizontal  plane  is 
to  be  utilized. 

In  calculating  the  illumination  due  to  any  type  of  lamp  at  a  given 
point  it  is  necessary  to  know  the  distribution  curve  of  the  lamp  used 
and  the  distance  to  the  point  illuminated.  The  approximate  illumi- 
nation of  a  plane  normal  to  the  rays  of  light  is  given  by  the  formula, 

_  _       c.p. 


h2  +  d2 

when  I  =  illumination  in  foot-candles. 

c.p.  =  candle-power  of  the  unit,  determined  from  the  distri- 
bution curve  of  the  lamp. 

h  =  distance  the,  lamp  is  mounted  above  the  ground,  in  feet, 
and  d  =  distance  from  the  base  of 
the  pole  supporting  the  lamp  to  the 
point  where  the  illumination  is  being 
considered,  Fig.  57. 

While  this  will  give  the  illumi- 
nation in  foot-candles,  the  nature  of     Fi&-  57-    street  Li^ht  illumination 

Diagram. 

the  lighting  cannot  be  decided  from 

this  alone,  but  the  total  amount  of  light  must  also  be  considered. 

Thus,  a  street  lighted  with  powerful  units  and  giving  a  minimum 


68 


ELECTRIC  LIGHTING 


Fig.  58. 


Ideal  Distribution  Curve  for  a  Street 
Light. 


illumination  of  .05  foot-candles  would  be  considered  better  illumi- 
nated than  one  having  smaller  units  so  distributed  as  to  give  the 
same  minimum  value.  .  \ 

*      \     A — 

Since  a  uniform  distribu- 
tion of  light  is  desirable,  for 
economic  reasons,  the  ideal 
distribution  curve  of  a  lamp 
for  street  lighting  would  be  a 
curve  which  shows  a  low  value 
of  candle-power  thrown  di- 
rectly downward,  but  with  the 
candle-power  increasing  as  we 
approach  the  horizon  tal .  Su ch 
an  ideal  distribution  curve  is  shown  in  Fig.  58. 

Actual  distribution  curves  taken  from  commercial  arc  lamps  are 
given  in  Fig.  59,  in  which 

Curve  A  shows  distribu- 
tion curve  for  a  9.6-ampere, 
gQ0  50°  40°    30<s  open,  direct-current  arc. 

Curve  B  shows  distribu- 
tion curve  for  a  6.6-ampere, 
D.C.  enclosed  arc 

Curve  C  shows  distribu- 
tion curve  for  a  7.5-ampere, 
A.C.  enclosed  arc. 

Globes  used  with  B  and 
C  are  opal  inner  globes, 
clear  outer  globes. 

Globes  used  with  A  are 
clear  outer  globes. 

A  street  reflector  was 
used  with  the  enclosed  arcs. 
Typical  curves  for 
flaming  and  luminous 
arc  lamps  are  shown  in 
Figs.  40,  43,  and  44. 

A  series  of  curves 
known  as  illumination 
curves  may  be  readily 
calculated  showing  the 

illumination  in  foot- 
Distribution  Curves  for  Commercial  Arc  ,,  .  ,. 
Lamps  Used  in  street  Lighting.                 candles  at  given  distance 


10° 


0° 


30° 


1300 


Fig.  59. 


ELECTRIC  LIGHTING 


69 


from  the  foot  of  the  pole  supporting  the  lamp.  Illumination  curves 
corresponding  to  the  distribution  curves  in  Fig.  59  are  given  in  Fig.  60 
where  A',  Bf  ,  andC"  correspond  to  A,  B,  and  C  in  Fig.  59.  These 
curves  correspond  to  actual  readings  taken  with  commercial  lamps. 
Similar  curves  for  incandescent  lamps  fitted  with  suitable  reflectors 
are  shown  in  Fig.  61.  A  value  oe  .03  foot-candles  is  about  the  min- 


Fig.  60.     Illumination  Curves  Drawn  to  Data  given  in  Fig.  59. 

imum  for  street  lighting.  Open  arcs  should  be  placed  at  least  25  feet 
above  the  ground;  30  to  40  feet  is  better,  especially  if  the  space  to  be 
illuminated  is  quite  open.  With  enclosed  arcs  it  is  often  advan- 
tageous to  place  them  as  low  as  18  to  20  feet  from  the  ground. 
Table  XVIII  gives  the  distance  between  lights  for  different  types 
of  arcs  for  fair  illumination. 

In  considering  the  type  of  arc  light  to  be  used  we  must  turn  to 
the  illumination  curves  as  shown  in  Fig.  60.  These  curves  show  that 
the  illumination  from  a  direct-current  open  arc  in  its  present  form 
is  superior  to  that  from  a  direct-current  enclosed  arc,  taking  the 


70 


ELECTRIC  LIGHTING 


TABLE  XVIII 


DISTANCE 

KIND  OF  LIGHT 

BETWEEN 

LIGHTS  PEK 

LIGHTS 

MILE 

6.6-ampere  enclosed  D.C.  arc  

340  feet 

15 

9.6-ampere  open  D.C.  arc  
6.6-ampere  enclosed  A  C.  arc 

315     " 
275     " 

17 
19 

6.6-ampere  open  D.C  arc 

260     " 

20 

same  amount  of  power,  in  the  vicinity  of  the  pole;  but  at  a  distance  of 
100  feet,  the  illumination  from  the  enclosed  arc  is  better.     This 

illumination  is  still  more  effective  on 
account  of  the  absence  of  such  strong 
light  as  is  given  by  the  open  arc  near 
the  pole.  The  pupil  of  the  eye  adjusts 
itself  to  correspond  to  the  brightest 


40'        so-         cor   light  in  the  field  of  vision,  and  we  are 
unable   to  see  as  well  in  the  dimly- 

Fig.  61.     Illumination    Curves   for    r    •,  ,     -,          ,•  i  ,1 

street  incandescent  Lamps.        lighted  section  as  when  the  maximum 

intensity  is   less.     The   characteristics 
of  the  open  and  enclosed  direct-current  arc  lamps  are  as  follows: 

The  mean  spherical  candle-power  and  energy  required  at  the  arc  are 
variable  with  the  open  arc. 

Fluctuations  of  light  are  marked,  due  to  wandering  of  the  arc,  flickering 
due  to  the  wind  and  lack  of  uniformity  of  the  carbons. 

Dense  shadows  are  cast  by  the  side  rods  and  the  lower  carbon,  while  the 
light  is  objectionably  strong  in  the  vicinity  of  the  pole. 

With  the  enclosed  arc  the  mean  spherical  candle-power  and  the  watts 
consumed  at  the  arc  are  fairly  constant. 

No  shadows  are  cast  by  the  lamps,  and  the  illumination  is  not  subject  to 
such  wide  variations.  The  enclosed  arc  is  much  superior  to  the  open  arc  using 
the  same  amount  of  energy.  This  applies  to  the  open  arc  as  it  is  now  used. 
With  proper  reflection  and  diffusion  of  the  light  such  as  might  be  accomplished 
by  extensive  or  special  shading,  we  ought  to  be  able  to  get  as  good  distribution 
from  the  open  arc  with  a  greater  total  amount  of  illumination. 

In  comparing  the  direct-current  with  the  alternating-current 
enclosed  arc,  we  see  that  the  direct-current  arc  gives  slightly  more  light 
than  the  alternating  lamp,  but  this  may  be  more  than  counterbalanced 
by  the  better  distribution  of  light  from  the  alternating-current  lamp. 
The  selection  of  A.C.  or  D.C.  enclosed  lamps  will  usually  depend  on 
other  conditions,  such  as  method  of  distribution  of  power,  efficient 
of  plant,  etc. 


ELECTRIC  LIGHTING 


71 


TABLE  XIX 
Street- Lamp  Data 


APPROX. 

APPROX. 

LAMP 

AMPERES 

WATTS  AT 
LAMP 

VALUE  op 

X    AS 

TERMINALS 

PROPOSED 

D.  C.  Series,  open  arc,  clear  globe 

J6.6 
{9.6 

330 
450 

3.5 
4 

D,  C.  Series,  enclosed,  clear  outer  globe 

J5.0 
]6.6 

370 
480 

3.5 
4 

(5.5 

345 

3 

Opalescent  inner  globe,  street  reflectors 

J6.6 

430 

3.5 

A.  C.  Series  as  above 

(7.5 

480 

4 

D.  C.  Series  "Magnetite" 

4.0 

310 

5.5 

The  question  of  street  lighting  has  been  given  considerable 
attention  by  the  National  Electric  Light  Association  and  this  society 
recommends  the  following  form  of  specification  for  street  lights: 

1.  Under  ordinary  conditions  of  street  lighting,  with  lamps  spaced  200 
to  600  feet  apart,  specifications  for  street  lamps  should  define  the  mean  illumi- 
nation thrown  by  the  individual  lamp,  in  position  in  the  street,  as  measured  at 
the  height  of  the  observer's  eye  and  perpendicular  to  the  rays,  at  some  point 
not  less  than  200  feet  nor  more  than  300  feet  distant,  along  a  level  street,  from 
a  position  immediately  below  the  lamp,  with  all  extraneous  light  screened  off 
and  with  no  reflection  from  surrounding  objects  not  forming  part  of  the  lamp 
equipment. 

2.  When  using  smaller  units  of  light,  such  as  series  incandescent  lamps 
spaced  shorter  distances  apart,  a  correspondingly  shorter  distance  from  the 
lamp  should  be  chosen  in  measuring  the  illumination. 

3.  The  lamp  contracted  for  should  give  a  mean  normal  illumination  at 
the  test  point  (selected  as  in  Sections  1  and  2)  not  less  than  the  illumination 
given  by  the  stationary  standard  incandescent  lamp  of  16  candle-power  at  1/X 
of  the  distance.     The  said  standard  incandescent  lamp  should  be  a  stand- 
ardized seasoned  lamp  having  a  determined  candle-power  in  a  fixed  direction. 

4.  When  the  lamp  tested  fluctuates  in  intensity,  a  number  of  observa- 
tions of  the  maximum  normal  illumination  should  be  made  at  a  distance  of  not 
less  than  200  feet  horizontally  from  beneath  the  lamp,  and  the  average  of  these 
measurements  should  be  taken  as  the  average  maximum  illumination.     A 
similar  number  of  observations  of  the  minimum  normal  illumination  should  be 
made,  the  average  of  which  should  be  taken  as  the  average  minimum  illumi- 
nation.    The  arithmetical  mean  of  the  said  average  maximum  and  minimum 
illuminations  should  be  taken  as  the  mean  normal  illumination  called  for  in 
Section  1. 

5.  A  reasonable  number  of  the  lamps  covered  by  the  contract  should 
be  tested. 

6.  For  measuring  the  mean  normal  illumination  of  a  lamp,  comparison 
with  the  standard  incandescent  lamp  may  be  made  either  with  a  suitable  portable 


72  ELECTRIC  LIGHTING 

photometer  or  with  a  reading  distance  instrument,  such  as  the  so-called  lumi- 
nometer. 

7.  The  unobstructed  mean  normal  illumination  must  not  be  less  at 
shorter  distances  than  at  the  point  of  test. 

8.  An  approximate  value  of  the  mean  normal  illuminations  thrown  by 
street  lamps  of  standard  manufacture,  at  horizontal  distances  within  the  200- 
300-foot  range,  hung  approximately  20  feet  above  the  observer's  eye,  may  be 
determined  from  Table  XIX. 

Series  incandescent  lamps  are  used  considerably  for  lighting 
the  streets  in  residence  sections  of  cities  or  where  shade  trees  make 
it  impracticable  to  use  arcs.  These  vary  in  candle-power  from  16 
to  50  or  even  higher,  and  are  usually  constructed  so  as  to  take  from 
two  to  four  amperes.  The  best  arrangement  of  these  is  to  mount 
them  on  brackets  a  few  feet  from  the  curb,  with  alternate  lamps  on 
opposite  sides  of  the  street.  The  distance  between  the  lamps  depends 
on  their  power.  50  candle-power  lamps  spaced  100  feet  between 
lamps,  give  a  minimum  illumination  of  .02  foot-candle.  25  candle- 
power  lamps  spaced  75  feet  between  lamps  will  serve  where  economy 
is  necessary. 

TABLE  XX 


PER  CENT 

Clear  glass                                                        

10 

Alabaster  glass                                          

15 

Opaline  glass                               

20-40 

25-30 

Opal  glass 

25-60 

Milkv  glass 

30-60 

Ground  glass 

24  4 

Opal  glass 

32  2 

Opaline  glass                                                          ... 

23 

SHADES  AND  REFLECTORS 

Lamps,  as  ordinarily  constructed,  do  not  always  give  a  suitable 
distribution  of  light,  while  the  intrinsic  brightness  is  often  too  high 
for  interior  lighting.  Shades  are  intended  to  modify  the  intensity 
of  the  light,  while  reflectors  are  used  for  the  purpose  of  changing  its 
direction.  Frequently  the  two  are  combined  in  various  ways.  Shades 
are  also  used  for  decorative  purposes,  but,  if  possible,  these  should 
be  of  such  a  nature  as  to  aid  illumination  rather  than  to  reduce  its 
efficiency. 


ELECTRIC  LIGHTING 


73 


A  considerable  amount  of  light  is  absorbed  by  the  material  used 
for  the  construction  of  shades.  Table  XX  shows  the  approximate 
amount  absorbed  by  some  materials. 

Of  the  great  number  of  styles  of  shades 
and  reflectors  in  use,  only  a  few  of  the  more 
important  will  be  considered  here. 

Frosted  Globes.  One  of  the  simplest 
methods  of  shading  incandescent  lamps  is  by 
the  use  of  frosted  bulbs.  These  serve  to 
reduce  the  intrinsic  brightness  of  the  lamp,  and 
should  be  freely  used  for  residence  lighting 
when  separate  shades  are  not  installed.  Frosted 
globes  are  also  used  in  connection  with  reflec- 
tors for  the  purpose  of  diffusing  the  reflected 
light.  The  McCreary  shade  as  shown  in  Fig. 
62,  is  an  example  of  such  a  combined  shade 
and  reflector.  Fig.  63  shows  the  distribution  Fig  62  Mccreary  Shade, 
curve  taken  from  an  incandescent  lamp  using 

a  McCreary  shade.  Fig.  64  shows  the  distribution  of  light  from  a  con- 
ical shade.  Fig.  56  shows  the  distribution  of  light  brought  about  by 
means  of  a  spiral  filament  and  a  reflector  as  used  in  the  Meridian  lamp. 


4-C5  C.P. 

Fig.  63.    Distribution  Curve  for  Incan- 
descent Lamp  Provided  with 
McCreary  Shade. 


Fig.  64.    Distribution  Curve  for  Incan- 
descent Lamp  Provided  with 
Conical  Shade. 


74  ELECTRIC  LIGHTING 

Holophane  Globes.  These  are  made  for  both  reflecting  and 
diffusing  the  light,  and  they  can  be  made  to  bring  about  almost  any 
desired  distribution  with  but  a  small  amount  of  absorption  of  light. 
These  consist  of  shades  of  clear  glass  having  horizontal  grooves 
forming  surfaces  which  change  the  direction  of  light  by  refraction  or 
total  reflection  as  is  necessary.  The  diffusion  of  light  is  effected  by 
means  of  deep,  rounded,  vertical  grooves  on  the  interior  surface  of 
the  globe.  While  these  globes  are  of  clear  glass  and  absorb  an  amount 
of  light  corresponding  to  clear  glass,  the  light  is  so  well  diffused  that 
the  filament  of  the  lamp  cannot  be  seen,  and  the  globe  appears  as  if 


Fig.  65.    Enclosed  Arc  Lamp  Fitted  with  Shade  and  Concentric  Diffuser. 

made  of  some  semi-transparent  material.  The  holophane  glassware 
is  made  in  a  large  variety  of  artistic  designs  and  for  all  types  of  in- 
candescent lamps.  By  the  proper  selection  of  a  reflector  the  dis- 
tribution of  the  light  of  the  unit  used  may  be  made  that  which  is  best 
suited  to  the  particular  case  of  lighting  in  hand.  Figs.  9,  13,  14, 
15, 16, 17,  and  18  give  some  idea  of  what  can  be  accomplished  by  these 
shades. 

Fig.  65  shows  an  enclosed  arc  lamp  fitted  with  a  shade  and  a 
concentric  diffuser.  The  effect  of  this  combination  is  best  shown 
in  Fig.  66.  Fig.  67  shows  the  change  in  the  illumination  curve  pro- 
.duced  by  such  shading.  Inverted  arcs  have  some  application  where 


ELECTRIC  LIGHTING 


75 


Fig.  66.     Diagram  Showing  Effect  of  the  Concentric  Diffuser. 

the  light  may  be  readily  reflected  and  diffused  as  in  lighting  large 
rooms  with  light  finish.  Reflectors  of  this  general  type  are  now  being 
manufactured  in  such  a  form  that  they  may  be  built  in  and  become 
part  of  the  ceiling  of  the  room  to  be  illuminated. 

LAMPS   WITH  OPAL  GLASS    SHADES. 


LAMPS  WITH    CONCENTRIC    LIGHT   DIFFUSERS. 


Fig.  67.     Illumination  Curves  for  Lamps  \rithand  without  Light  Diffusers. 


76  ELECTRIC  LIGHTING 

Opal  Enclosing  Globes.  The  use  of  opal  enclosing  globes  is 
recommended  for  arc  lamps  used  for  street  lighting  for  the  reason 
that  they  change  the  distribution  of  the  light  so  that  it  covers  a  greater 
area,  and  the  light  is  so  diffused  as  to  obliterate  shadows  in  the  vicinity 
of  the  lamp.  Table  XXI  gives  the  efficiency  of  different  globe  com- 
binations for  street  lighting  assuming  the  opal  inner  and  the  clear 
outer  globes  as  100%. 

TABLE  XXI 


O  pal  enclosing  and  clear  outer 

Clear          "  "    clear      "      . 

"    opal       "      . 

Opal          "          "    opal       "      . 


100      per  cent 
91.2 
85.1 

82.7 


PHOTOMETRY 

Photometry  is  the  art  of  comparing  the  illuminating  properties 
of  light  sources,  and  forms  one  branch  of  scientific  measurement. 
Its  use  in  electric  illumination  is  to  determine  the  relative  values  of 
different  types  of  lamps  as  sources  of  illumination,  together  with  their 
efficiency;  also  by  means  of  the  principles  of  photometry,  we  are  able 
to  study  the  distribution  of  illumination  for  any  given  arrangement 
of  light  sources. 

LIGHT  STANDARDS 

Inasmuch  as  sources  of  light  are  compared  with  one  another  in 
photometry,  we  must  have  some  standard,  or  unit,  to  which  all  light 
sources  are  reduced.  This  unit  is  usually  the  candle-power  and  the 
rating  of  most  lamps  is  given  in  candle-power. 

While  the  candle-power  remains  the  unit  and  is  based  on  the 
standard  English  candle,  other  light  standards  have  been  introduced 
and  are  much  more  desirable. 

The  English  Candle.  The  English  candle  is  made  of  spermaceti 
extracted  from  crude  sperm  oil,  with  the  addition  of  a  small  quantity 
of  beeswax  to  reduce  the  brittleness.  Its  length  is  ten  inches,  and  its 
diameter  .9  inch  at  the  bottom  and  .8  inch  at  the  top,  and  its  weight 
is  one-sixth  of  a  pound.  Great  care  is  taken  in  the  preparation  of 
the  wick  and  spermaceti.  This  candle  burns  with  a  normal  height 
of  flame  of  45  millimeters  and  consumes  120  grains  per  hour  when 


ELECTRIC  LIGHTING  77 

burning  in  dry  air  at  normal  atmospheric  pressure.  Under  these 
conditions,  the  light  given  by  a  single  candle  is  one  candle-power. 

When  used  for  measurements,  the  candle  should  be  allowed  to 
burn  at  least  fifteen  minutes  before  taking  any  readings.  At  the  end 
of  this  period  the  wick  should  be  trimmed,  if  necessary,  and  when  the 
flame  height  reaches  45  millimeters,  readings  can  be  taken.  The 
candle  should  not  require  trimming  when  the  proper  height  of  flame 
has  been  reached.  It  is  best  to  weigh  the  amount  of  material  con- 
sumed by  balancing  the  candle  on  a  properly  arranged  balance  when 
the  first  reading  is  taken,  and  again  balancing  at  the  end  of  a  suitable 
period — ten  to  fifteen  minutes.  The  candle-power  of  the  unit  is 
then,  practically,  directly  proportional  to  the  amount  of  the  material 
consumed. 

The  objections  to  the  candle  as  a  unit  are  that  it  burns  with  an 
open  flame  which  is  subject  to  variation  in  height  and  to  the  effect  of 
air  currents.  The  color  of  the  light  is  not  satisfactory,  being  too 
rich  in  the  red  rays,  and  the  composition  of  the  spermaceti  is  more  or 
less  uncertain. 

The  German  Candle  is  made  of  very  pure  paraffine,  burns 
with  a  normal  flame  height  of  50  millimeters,  and  is  subject  to  the 
same  disadvantages  as  the  English  candle.  It  may  be  necessary  to 
trim  the  wick  to  keep  the  flame  height  at  50  millimeters.  The  light 
given  is  a  trifle  greater  than  for  the  spermaceti  candle. 

The  Carcel  Lamp  is  built  according  to  very  careful  specifications 
and  burns  colza  (rape  seed)  oil.  It  has  been  used  to  a  large  extent 
in  France,  but  its  present  application  is  limited. 

The  Pentane  Lamp  is  a  specially  constructed  lamp  burning 
pentane,  prepared  by  the  distillation  of  gasoline  between  narrow 
limits  of  temperature.  This  standard  is  not  extensively  used. 

The  Amyl  Acetate  Lamp.  This  lamp,  known  also  as  the  Hefner 
lamp,  is  at  present  the  most  desirable  standard.  It  is  a  lamp  built 
to  very  careful  specifications,  especially  with  regard  to  the  dimension 
of  the  wick  tube.  It  burns  pure  amyl  acetate  and  the  flame  height 
should  be  40  millimeters.  This  flame  height  must  be  very  carefully 
adjusted  by  means  of  gauges  furnished  with  the  lamp.  Amyl  acetate 
is  a  colorless  hydrocarbon  prepared  from  the  distillation  of  amyl 
alcohol  obtained  from  fusel  oil,  with  a  mixture  of  acetic  and  sulphuric 
acids,  or  by  distillation  of  a  mixture  of  amyl  acetate,  sulphuric  acid, 


78 


ELECTRIC  LIGHTING 


and  potassium  acetate.  It  has  a  definite  composition,  and  must  be 
pure  for  this  use. 

The  most  serious  disadvantage  of  this  standard  is  the  color  of 
the  light,  inasmuch  as  it  has  a  decidedly  red  tinge  and  is  not  readily 
compared  with  whiter  lights.  Its  value  is  affected  somewhat  by  the 
moisture  in  the  air  and  the  atmospheric  pressure,  but  it  excels  all  other 
standards  in  that  it  is  quite  readily  reproduced. 

Table  XXII  gives  the  value  of  the  candle-power  units  of  different 
laboratories  in  terms  of  the  unit  of  the  Bureau  of  Standards  and  also 
the  values  of  the  units  of  the  Carcel  and  Vernon-Harcourt  in  terms  of 
the  Hefner,  as  accepted  by  the  International  Photometric  Commission. 

TABLE  XXII 
Photometric  Units 


Bureau  of  Standards  Unit,  United"  States 

1.000 

Reichsanstalt  Unit,  Germany 

0.998 

X  0.88 

National  Physical  Laboratory  Unit,  England 

0.984 

Laboratoire  Central  Unit,  France 

0.982 

CARCEL 

HEFNER 

VERNON- 
HARCOURT 

Carcel 
Hefner 
Vernon-Harcourt  (pentane) 

1.00 
0.0930 
1.020 

10.75 
1.00 
10.95 

0.980 
0.0915 
1.0 

The  above  values  are  at  a  barometric  pressure  of  760  mm.  of  mercury  and  a  humid- 
ity for  the  Carcel  and  Vernon-Harcourt  standards  of  10.0  liters  of  water  per  cubic  meter 
of  dry  air.  The  humidity  for  the  hefner  unit  is  8.8  liters  of  water  to  one  cubic  meter 
of  dry  air. 

Working  Standards.  Incandescent  Lamp.  The  units  just  de- 
scribed, together  with  some  others,  form  reference  standards,  but  an 
incandescent  lamp  is  generally  used  as  the  working  standard  in  all 
photometers.  An  incandescent  lamp,  when  used  for  this  work,  should 
be  burned  for  about  two  hundred  hours,  or  until  it  has  reached  the 
point  in  the  life  curve  where  its  value  is  constant,  and  it  should  then 
be  checked  by  means  of  some  standard  when  in  a  given  position  and 
at  a  fixed  voltage.  It  then  serves  as  an  admirable  working  standard 
if  the  applied  voltage  is  carefully  regulated.  Two  such  lamps  should 
always  be  used — the  one  to  serve  as  a  check  on  the  other;  the  checking 
lamp  to  be  used  for  very  short  intervals  only. 


ELECTRIC  LIGHTING 


79 


PHOTOMETERS 

Two  light  sources  are  compared  by  means  of  a  photometer 
which;  in  one  of  its  simplest  forms,  consists  of  what  is  known  as  a 
Bunsen  screen  mounted  on  a  carriage  between  the  two  lights  being 
compared,  with  its  plane  at  right  angles  to  a  line  passing  through 
the  light  sources,  and  arranged  with  mirrors  or  prisms  so  that  both 
sides  of  the  screen  may  be  observed  at  once.  The  Bunsen  screen 
consists  of  a  disk  of  paper  with  a  portion  of  either  the  center,  or  a 
section  around  the  center,  treated  with  paraffine  so  as  to  render  it 
translucent.  If  the  light  falling  on  one  side  of  this  screen  is  in  ex- 
cess, the  translucent  spot  will  appear  dark  on  that  side  of  the  screen 
and  light  on  the  opposite  side. 
Care  must  be  taken  to  see  that 
the  two  sides  of  the  screen  are 
exactly  alike,  otherwise  there  will 
be  an  error  introduced  in  using 
the  screens.  It  is  well  to  reverse 
the  screen  and  check  readings 
whenever  a  new  lot  of  lamps  are 
to  be  tested.  When  the  light 
falling  on  the  two  sides  of  the 
screen  is  the  same,  the  trans- 
parent spot  disappears.  The 
values  of  the  two  light  sources  are 
then  directly  proportional  to  the 
square  of  their  distances  from  the 
screen.  As  an  example,  consider  a  16  candle-power  lamp  being 
compared  with  a  standard  candle  on  a  photometer  with  a  300-centi- 
meter bar.  Say  the  translucent  spot  disappears  when  the  screen 
is  distant  60  centimeters  from  the  standard  candle,  we  then  have 
the  proportion, 

x  :  1  -  (240)2  :  (60)2  ==  16  :  1, 

showing  that  the  lamp  gives  16  candle-power. 

The  above  law  is  known  as  the  law  of  inverse  squares,  and  holds 
true  only  when  the  dimensions  of  the  light  sources  are  small  com- 
pared with  the  distance  between  them,  and  when  there  are  no  reflecting 
surfaces  present  as  when  the  readings  are  taken  in  a  dark  room. 


Fig.  68.      Proof  of   the    Law    of    Inverse 
Squares  by  the  Method  of  Con- 
centric Spheres. 


80 


ELECTRIC  LIGHTING 


The  proof  that  the  light  varies  inversely  with  the  square  of  dis- 
tance from  the  source  is  as  follows: 

Consider  two  spherical  surfaces,  Fig.  68,  illuminated  by  a  source 
of  light  at  the  center.  The  same  quantity  of  light  falls  on  both  sur- 
faces. 

Area  of  8    =  lirR2  sq.  ft.  (R  is  in  feet.) 
Area  of  $x  =  4rn-R21  sq.  ft. 

Let  Q  =  total  quantity  of  light  and  q  =  light  falling  on  unit 
surface.  Then, 

Q 


Q 


Q 


Q 


Fig.  69  shows  the  relation  in  another  way.  The  area  of  C.  dis- 
tant two  units  from  the 
source  of  light  A,  is  four 
times  that  of  B  which  is 
distant  one  unit. 

The  Lummer=Brodhun 
Photometer.  In  addition 
to  the  Bunsen  screen  de- 
scribed, there  are  several 
other  forms  of  photom- 
eters, the  most  important 

Fig.  69.     Proof  of  the  Law  of  Inverse  Sqiiares  by      »       ,  .  ,     .  T 

Method  of  Screen  Shadow.  of  wnicn  is  the  Lummer- 

Brodhun.     The    essential 

feature  of  this  instrument  is  the  optical  train  which  serves  to  bring 
into  contrast  the  portions  of  the  screen  illuminated  by  the  two  sources 
of  light.  Referring  to  Fig.  70  the  screen  S  is  an  opaque  screen  which 


ELECTRIC  LIGHTING 


81 


reflects  the  light  falling  upon  it  from  Z,  to  the  mirror  M,  when  it  is 
again  reflected  to  the  pair  of  glass  prisms  A,  B.  The  surfaces  sr  are 
ground  to  fit  perfectly  and  any  light  falling  on  this  surface  will  pass 
through  the  prisms.  Light  falling  on  the  surface  ar  or  bs  will  be  re- 
flected as  shown  by  the  arrows.  We  see  then  that  the  light  from  L, 
which  falls  on  ar  and  bs,  is  reflected  to  the  eye  piece  or  telescope  T, 
while  that  falling  on  sr  is  transmitted  to  and  absorbed  by  the  black 
interior  of  the  containing  box.  Likewise,  the  light  from  the  screen 


Fig.  70.     Diagram  of  Lummer-Brodhun  Screen. 

L1  is  reflected  by  the  screen  M^  to  the  pair  of  prisms  A,  B.  The 
rays  falling  on  the  surface  sr  pass  through  to  the  telescope  T,  while 
the  rays  falling  on  ar  and  bs  are  reflected  and  absorbed  by  the  black 
lining  of  the  case.  The  field  of  light,  as  then  viewed  through  the 
telescope,  appears  as  a  disk  of  light  produced  by  the  screen  Lv  sur- 
rounded by  an  annular  ring  of  light  produced  by  L.  When  the 
illumination  on  the  two  sides  of  the  screen  is  the  same,  the  disk  and 
ring  appear  alike  and  the  dividing  circle  disappears. 

In  using  this  screen,  it  is  mounted  the  same  as  the  Bunsen  screen 
and  readings  are  taken  in  the  same  manner.  The  screen  and  prisms 
are  arranged  so  that  they  can  be  reversed  readily  and  two  readings 


82 


ELECTRIC  LIGHTING 


should  always  be  taken  to  compensate  for  any  inequalities  in  the  sides 
of  the  screen  and  the  reflecting  surfaces,  a  mean  of  the  two  readings 


Fig.  71.     Complete  Photometer  with  Liunmer-Broclhun  Screen. 

serving  as  the  true  reading.     This  form  of  screen  is  used  when  es- 
pecially accurate  comparisons  are  required. 

Fig.  71  shows  a  complete  photometer  with  a  Lummer-Brodhun 
screen,  while  Fig.  72  shows  a  Bunsen  screen  and  sight  box.  In  Fig. 
71,  the  lamps  are  shaded  by  means  of  curtains  so  as  to  leave  only  a 


ELECTRIC  LIGHTING 


83 


smtill  opening  toward  the  screen.  If  the  lights  are  properly  screened 
photometric  measurements  may  be  made  in  rooms  having  light- 
colored  walls. 


Fig.  72.     Bunsen  Screen  and  Sight  Box. 

The  Weber  Photometer.  As  an  example  of  a  portable  type  of 
photometer,  we  have  the  Weber.  This  photometer,  shown  in  Fig.  73, 
is  very  compact  and  is  especially  adapted  to  measuring  intensity  of 
illumination  as  well  as 
the  value  of  light  sources ; 
it  may  be  used  for  ex- 
ploring- the  illumination 
of  rooms  or  the  lighting 
of  streets. 

This  apparatus  con- 
sists of  a  tube  A,  Fig.  74, 
which  is  mounted  hori- 
zontally and  contains  a 
circular,  opal  glass  plate 
/,  which  is  movable  by 
means  of  a  rack  and 
pinion.  To  this  screen  is 
attached  an  index  finger 
which  moves  over  a  scale 
attached  to  the  outside  of 
the  tube.  A  lamp  L, 

burning  benzine,  is  mounted  at  the  end  of  this  tube.     The  benzine 
used  should  be  as  pure  as  possible,  and  the  flame  height  should  be 


Fig.  73.     Weber  Portable  Photometer. 


84 


ELECTRIC  LIGHTING 


carefully  adjusted  to  20  mm.  when  taking  readings.  At  right  angles 
to  the  tube  A  is  mounted  the  tube  B  which  contains  an  eye  piece  at 
0,  a  Lummer-Brodhun  contrast  prism  at  p,  and  a  support  for  opal  or 
colored  glass  plates  at  g. 

Operation.  The  tube  B  is  turned  toward  the  source  of  light  to 
be  measured,  the  distance  from  the  light  to  the  screen  at  g  being  noted. 
The  light  from  this  source  is  diffused  by  the  screen  at  g,  while  that 
from  the  standard  is  diffused  by  the  screen  /.  By  moving  (he  screen 
/,  the  light  falling  on  either  side  of  the  prism  p  can  be  equalized. 
The  value  of  the  unknown  source  can  be  determined  from  the  reading 
of  the  screen  /,  the  photometer  having  previously  been  calibrated  by 

means  of  a  standard  lamp 
in  place  of  the  one  to  be 
measured.  The  calibra- 
tion may  be  plotted  in  the 
form  of  a  curve  or  it  may  be 
denoted  by  a  constant  C, 
when  we  have  the  formula, 


0 


n 


r  =  c  ~ 

C  corresponds  to  a  par- 
ticular plate  at  g,  I  =  dis- 
tance of  screen  /  from  the 
benzine  lamp,  and  L  =  dis- 
tance from  the  screen  g  to 
the  light  source  being 
measured.  Screens  of  dif- 
ferent densities  may  be 
used  at  g,  depending  on 
the  strength  of  the  light  source. 

When  used  for  measuring  illumination,  a  white  screen  is  used 
in  connection  with  this  photometer.  The  screen  is  mounted  in  front 
of  the  opening  at  g}  and  turned  so  that  it  is  illuminated  by  the  source 
being  considered.  Readings  of  the  screen  /  are  taken  as  before.  A 
calibration  curve  is  plotted  for  the  instrument,  using  a  known  light 
source  at  a  known  distance  from  the  white  screen  when  the  instru- 
ment is  mounted  in  a  dark  room. 


Fig.  74.     Diagram  of  Weber  Photometer. 


ELECTRIC  LIGHTING 


85 


Portable  Photometers,  There  is  a  large  variety  of  portable 
photometers  available  and  giving  more  or  less  satisfactory  results. 
An  instrument  especially  designed  with  a  view  to  portability  and  to 
overcoming  some  of  the 
defects  of  instruments 
already  on  the  market 
has  recently  been  intro- 
duced. The  instrument 
referred  to  is  called  a 
Universal  photometer  but 
it  is  more  commonly 
known  as  the  Sharp- 
Millar  photometer  from  the  names  of  its  inventors.  Views  of  this 
instrument  are  shown  in  Figs.  75  and  76.  It  is  adapted  to  the  meas- 
urement of  the  intensity  of  light  sources  as  well  as  to  the  illumination 
at  any  point,  as  is  the  Weber  photometer.  The  photometer  screen 
or  photometric  device  is  shown  at  B,  and  consists  of  a  special  form  of 


Fig.  75.     Universal  Photometer. 


Fig.  76.     Sectional  View  of  Universal  Photometer. 

Lummer-Brodhun  optical  screen.  A  standardized  incandescent 
lamp  C  is  used  as  the  photometric  standard  and  this  may  be  con- 
nected to  a  battery,  or  be  adapted  to  use  on  the  mains  supplying  the 
lamps  in  the  room  where  measurements  are  to  be  taken.  All  stray 
light  is  carefully  screened  from  the  interior  of  the  box  by  a  series  of 
screens  G.  The  instrument  scale  is  calibrated  in  foot-candles  and  in 
candle-powers. 


86  ELECTRIC  LIGHTING 

When  illumination  is  to  be  measured,  a  specially  selected  trans- 
luscent  screen  is  placed  at  A  and  the  illumination  of  this  plate,  which 
is  placed  at  the  point  and  in  the  plane  where  the  value  of  the  illumi- 
nation is  desired,  is  reflected  to  the  photometric  device  by  the  mirror 
at  H.  A  second  plate  K  is  mounted  so  as  to  be  illuminated  by  the 
standard  lamp  and  the  photometer  is  balanced  by  making  the  illumi- 
nation of  A  and  K  the  same.  When  the  intensity  of  a  light  source 
is  to  be  determined,  the  screen  at  A  is  replaced  by  a  small  aperture 
and  a  diffusing  surface  I  is  put  in  place  of  the  mirror  H.  The  illumi- 
nation of  7  is  now  compared  with  the  illumination  of  K,  and  when 
the  two  are  made  equal,  the  photometer  reads  the  candle-power  of  the 
light  source,  or  some  multiple  of  this  candle-power.  The  range  of 
this  instrument  is  increased  by  the  use  of  suitably  arranged  absorbing 
screens  which  may  be  readily  inserted  or  removed,  and  as  ordinarily 
equipped,  the  range  in  foot-candles  is  approximately  from  .004  to 
2,000.  The  variety  of  uses  which  can  be  made  of  such  a  photometer 
is  large,  and  some  idea  of  its  portability  can  be  obtained  from  the 
dimensions  of  the  box,  24"  x  4J"  x  5",  and  its  weight,  fully  equipped, 
of  8  pounds.  It  is  very  accurate  considering  its  compactness. 

Integrating  Photometers.  Matthews.  This  photometer  is  used 
to  some  extent  and  a  very  good  idea  of  its  construction  can  be  ob- 
tained from  Fig.  77.  By  means  of  a  system  of  mirrors,  the  light 
given  by  the  lamp  in  several  directions  may  be  integrated  and  throwrn 
on  the  photometer  screen  for  comparison  with  the  standard,  the  result 
giving  the  mean  spherical  candle-power  from  one  reading.  By  cover- 
ing all  but  one  pair  of  screens,  the  light  given  in  any  one  direction 
is  easily  determined. 

Another  type  of  integrating  photometer  is  known  as  the  inte- 
grating sphere  or  globe  photometer.  If  a  light  source  is  placed  within 
a  sphere,  the  interior  walls  of  which  are  coated  with  a  white  diffusing 
surface,  the  illumination  of  that  surface  at  any  point  is  due  partly 
to  the  light  falling  on  it  directly,  and  partly  to  the  light  reflected  from 
the  remainder  of  the  surface  of  the  sphere.  The  reflected  light  is 
proportional  to  the  total  flux  of  light  from  the  light  source  and  so, 
if  the  direct  light  is  screened  from  the  point  considered,  its  illumina- 
tion is  proportional  to  the  total  flux  of  light,  and  hence  to  the  mean 
spherical  candle-power  of  the  light  source. 

The  practical  application  of  this  principle  is  to  so  arrange  our 


ELECTRIC  LIGHTING 


87 


properly  coated  sphere  that  the  lamp  to  be  tested  may  be  readily 
inserted;  to  replace  a  small  portion  of  the  sphere  by  a  piece  of  un- 
polished white  glass;  to  shut  off  the  direct  rays  of  the  lamp  to  be 


Fig.  77.     Integrating  Photometer. 

measured  from  this  glass  surface;  and  to  so  mount  a  photometer 
screen  and  standard  lamp  that  the  illumination  of  the  glass  section 
can  be  measured.  Under  these  conditions  the  illumination  of  the 
glass  screen  is  proportional  to  the  mean  spherical  candle-power  of  the 
lamp  under  test.  A  substitution  method  is  used  in  practice.  A 


88 


ELECTRIC  LIGHTING 


standardized  lamp  of  the  general  type  of  the  one  to  be  tested  is  mounted 
in  the  sphere  and  the  constant  of  the  instrument  for  this  type  of  lamp 
is  determined.  The  unknown  lamps  are  then  put  in  place  and  their 
candle-power  is  readily  determined,  once  the  constant  of  the  instru- 
ment is  known.  Figs.  78  and  79  give  some  views  of  the  integrating 


Fig.  78.     Eighteen -Inch  Integrating  Sphere  Equipped  with  Photometer. 

sphere  and  indicate  the  range  of  the  sizes  in  which  it  may  be  con- 
structed. 

INCANDESCENT  LAMP  PHOTOMETRY 

Apparatus.  Some  sort  of  screen,  either  the  Bunsen  type  or  the 
Lummer-Brodhun  screen  preferred,  should  be  mounted  on  a  carriage 
moving  on  a  suitable  scale,  and  the  lamp  holders,  one  for  the  standard, 
the  other  for  the  lamp  to  be  tested,  are  mounted  at  the  ends  of  this 
scale.  There  are  several  types  of  so-called  station  photometers 
arranged  so  as  to  be  very  convenient  for  testing  incandescent  lamps. 
Fig.  80  shows  one  form  of  station  photometer  manufactured  by 
Queen  &  Co.  The  controlling  rheostats  and  shielding  curtains  are 
not  shown  here.  Fig.  81  shows  a  form  of  portable  photometer  for 


ELECTRIC  LIGHTING 


89 


incandescent  lamps.  The  length  of  scale  should  not  be  less  than 
100  centimeters,  and  150  to  200  centimeters  is  preferred.  This  scale 
may  be  divided  into  centimeters  or,  for  the  purpose  of  doing  away 
with  much  of  the  calculation,  the  scale  may  be  a  proportional  scale. 
This  scale  is  based  on  the  law  of  inverse  squares  and  reads  the  inverse 
ratio  of  the  squares  of  the  distances  from  the  two  lights  being  compared. 


Fig.  79.    Interior  of  80-Inch  Integrating  Sphere. 

If  the  standard  used  always  has  the  same  value,  the  scale  may  be 
made  to  read  in  candle-powers  directly. 

For  mean  horizontal  candle-power  measurements,  the  lamp 
should  be  rotated  at  180  revolutions  per  minute,  when  mounted  in  a 
vertical  position. 

For  distribution  curves  a  universal  lamp  holder  which  will 
allow  the  lamp  to  be  placed  in  any  position,  and  which  indicates  this 
position,  is  used. 

^or  mean  spherical  candle-power,  the  following  method  is  used 
when  the  Matthews  photometer  is  not  available : 


90  ELECTRIC  LIGHTING 

The  lamp  is  placed  in  an  adjustable  holder  and  readings  taken 
with  the  lamp  in  thirty-eight  positions,  as  follows : 

The  measurement  of  the  spherical  intensity.  For  convenience 
the  tip  of  the  lamp  and  its  base  may  be  termed  the  north  and  south 
poles  respectively. 


Fig.  80.     Station  Photometer. 

The  mean  of  13  readings  taken  at  intervals  of  30°,.  is  taken  to  give  the 
mean  horizontal  candle-power. 

Beginning  again  at  0°  azimuth,  thirteen  readings  are  made  in  the  prime 
meridian  or  vertical  circle,  the  interval  again  being  30°,  and  the  last  reading 
checking  the  first. 


Fig.  81.     Portable  Photometer  for  Incandescent  Lamps. 

It  will  be  noticed  that  four  readings,  two  being  check  readings,  have  been 
made  at  0°  azimuth  in  each  case.  The  mean  of  the  four  is  taken  as  the  standard 
reading,  it  being  the  value  of  the  intensity,  in  this  position,  should  the  lamp  be 
used  as  a  standard. 

Additional  sets  of  thirteen  readings  each — the  last  reading  checking  the 
first  one — are  similarly  made  on  each  of  the  vertical  circles  through  45°,  90°, 
and  135°  azimuth. 


ELECTRIC  LIGHTING  91 

In  combining  the  readings  for  the  mean  spherical  intensity,   a  note  is 
taken  of  the  repetitions. 

Neglecting  the  repetitions,  which   may  also  be  omitted  in  part,  in  the 
practice  of  the  method,  there   remain  thirty-eight  points,  as  follows: 

DISTRIBUTED 
VALUES 

The  mean  of  four  measurements  at  the  north  pole  of  the  lamp 1 

Four  measurements  on  each  of  the  vertical  circles  through  0°  and  90° 

azimuth  at  vertical  circle  readings  of  60°,  120°,  240°,  and  300°... .       8 
Four  measurements  on  each  of  the  vertical  circles  through  0°,  45°,  90°, 

and  135°  azimuth  at  vertical  circle  readings  of  30°,  150°,  210°,  and 

330° 16 

Twelve  measurements  30°  apart  at  the  equator 12 

Four  null  values  at  the  south  pole  of  lamp 1 

Total  number  of  effective  measurements 38 

The  points  thus  laid  off  on  the  reference  sphere  are  approximately  equi- 
distant, being  somewhat  closer  together  at  the  equator  than  at  the  poles. 

When  the  lamp  is  rotated,  readings  are  taken  for  each  15°  or  30° 
in  inclination,  from  0°  to  90°,  and  from  0°  to  270°.  These  are  inte- 
grated values  for  their  corresponding  parallels  of  latitude  on  the  unit 
sphere. 

The  mean  spherical  candle-power  from  these  readings  may  best 
be  obtained  by  plotting  a  distribution  curve  from  the  readings,  deter- 
mining the  area  of  this  closed  curve  by  means  of  a  planimeter  and 
taking  the  radius  of  an  equivalent  circle  as  the  value  for  the  mean 
spherical  candle-power. 

The  Rousseau  diagram  may  be  used  for  determining  the  mean 
spherical  candle-power  of  a  lamp  when  its  vertical  distribution  curve 
is  known.  Fig.  82  shows  such  a  diagram  made  up  for  a  gem  lamp 
with  a  bowl  reflector.  Where  the  horizontal  distribution  curve  of  the 
lamp  is  net  uniform  the  values  for  the  vertical  distribution  curve 
should  be  taken  with  the  lamp  rotating  so  as  to  give  average  values 
at  each  angle.  One-half  of  the  distribution  curve  is  drawn  to  scale  A 
and  a  circle  B  is  drawn  with  the  source  of  light  0  as  a  center.  Radii 
C  are  drawn  at  equal  angles  about  the  light  source  and  extended  until 
they  intersect  the  circle  B.  The  points  of  intersection  of  these  lines 
with  the  circle  are  projected  upon  the  straight  line  D  E.  Distances 
from  this  line  are  laid  off  on  the  verticals  F  equal  to  the  distances 
from  the  center  of  the  circle  to  the  points  where  the  corresponding 
radii  cut  the  distribution  curve.  The  area  enclosed  between  the 
straight  line  D  E  and  a  curve  drawn  through  the  points  just  deter- 
mined, G  H,  divided  by  the  base  line,  is  equal  to  the  mean  spherical 


92 


ELECTRIC  LIGHTING 


candle-power  of  the  lamp.  If  the  mean  candle-power  ot  the  lamp 
within  a  certain  angle  is  desired,  it  is  only  necessary  to  find  the  area 
of  the  diagram  within  the  space  indicated  by  that  angle  and  divide 
by  the  corresponding  base. 


„,     Vertical  o 

^--r-^^™^:--:r   -y-  -y 
1        -\— -/~    »          >-"^^  \/ /  'i  *  \V/* xv       r "--..'          ; 

W|^7^>?^^^VN>x"/^ 


•-V^0 


n  I 
'I  I 
«'  I 
n  i 
li 


/         -    '  Jr~*       \ 


;;!  i 


\     X 


C 


/^ 


80 


6 

60 


40 


20 


K'- 


O°/Oa20°30°     ?5^          60°  75°  90°  /OS  J3O°        /35°    /50°/65°/aOa 

Fig.  82.    Rousseau  Diagram  for  Gem  Lamp  with  Bowl  Reflector. 

In  all  tests  the  voltage  of  the  lamp  must  be  very  closely  regulated. 
A  storage  battery  forms  the  ideal  source  of  current  for  such  purposes. 
•In  testing  incandescent  lamps,  a  standard  similar  to  the  lamp  being 
tested  is  desirable  and  it  should,  preferably,  be  connected  to  the  same 
leads.  Any  variation  in  the  voltage  of  the  mains  then  affects  both 
lamps  and  the  error  introduced  is  slight. 


ELECTRIC  LIGHTING  03 

ARC  LIGHT  PHOTOMETRY 

Owing  to  the  variation  of  the  amount  of  light  given  out  by  an 
arc  lamp  in  one  direction  at  any  time,  due  to  variation  of  the  qualities 
of  the  carbons,  position  of  the  arc,  and  also  on  account  of  the  color 
of  the  light,  etc.,  the  photometry  of  arc  lamps  is  much  more  difficult 
than  that  of  incandescent  lamps.  The  curves  shown  in  Figs.  33  and 
34  are  average  distribution  curves  taken  from  several  lamps  and  will 
vary  considerably  for  any  one  lamp.  If  the  arc  is  enclosed,  this 
variation  is  not  so  great. 

The  working  standard  should  be  an  incandescent  lamp  run  at 
a  voltage  above  the  normal  so  that  the  quality  of  the  light  will  com- 
pare favorably  with  that  of  the  arc.  Since  an  incandescent  lamp 
deteriorates  rapidly  when  run  at  over  voltage,  the  standard  can  be 
used  only  for  short  intervals  and  must  be  frequently  checked. 

Since  an  arc  lamp  can  be  mounted  in  one  position  only,  mirrrors 
must  be  used  to  obtain  distribution  curves.  A  mirror  is  used  mounted 
at  45°  with  the  axis  of  the  photometer,  and  arranged  so  as  to  reflect 
the  arc  when  in  different  positions.  A  mirror  absorbs  a  certain  per 
cent  of  the  light  falling  upon  it  and  this  percentage  must  be  deter- 
mined by  using  lamps  previously  standardized.  The  length  of  the 
photometer  bar  must  include  the  distance  from  the  mirror  to  the  arc. 

The  Weber  photometer  is  well  adapted  to  arc-light  measure- 
ments inasmuch  as  appropriate  screens  may  be  used  to  cut  down 
the  intensity  of  the  light. 

A  special  form  of  the  Matthews  photometer  is  also  used  for 
testing  arc  lamps. 

For  the  comparison  of  the  illumination  from  arc  lamps  as  in- 
stalled in  service,  an  instrument  known  as  an  illuminometer  is  some- 
times used.  This  consists  of  a  light  wooden  box,  readily  portable, 
having  a  black  interior  and  arranged  with  two  openings.  One 
of  these  openings  is  for  the  purpose  of  admitting  light  from  the  source 
being  considered,  to  a  printed  card.  The  other  opening  is  for  the 
purpose  of  viewing  this  card  when  illuminated  by  the  light  source. 
The  printing  on  the  card  is  made  up  from  type  of  different  sizes,  and 
the  smallest  size  which  is  legible,  together  with  the  distance  from  the 
light  source,  is  noted.  Another  method  of  application  is  to  select 
some  definite  size  of  type  and  then  to  move  the  instrument  from  the 


94  ELECTRIC  LIGHTING 

light  source  to  a  point  where  this  type  is  just  legible  and  note  the  dis- 
tance. From  similar  measurements  taken  on  different  lamps  a  good 
comparison  may  be  obtained.  Such  an  instrument  is  very  convenient 
to  use,  and  results  obtained  by  different  observers  check  very  closely. 

The  flicker  photometer  is  used  for  the  comparison  of  different 
colored  lights,  the  basis  for  comparison  being  that  each  light,  though 
different  in  color,  shall  produce  light  sensations  equally  intense  for  the 
purpose  of  distinguishing  outlines.  It  consists,  in  one  form,  of  an 
arrangement  by  means  of  which  a  sectored  disk  is  rotated  in  front  of 
each  light  source,  these  disks  being  so  arranged  that  the  light  from 
one  source  is  cut  off  while  the  other  falls  on  the  screen,  and  vice  versa, 
any  form  of  screen  being  used  for  making  the  comparison.  The  disks 
must  be  revolved  at  such  a  rate  that  the  light,  viewed  from  the  oppo- 
site side,  will  appear  continuous.  When  the  illumination  of  the  two 
sides  of  the  screen,  under  these  conditions,  is  not  the  same,  there  will 
be  a  perceptible  flicker  and  the  screen  should  be  so  adjusted  that  this 
flicker  disappears.  The  value  of  the  light  source  can  then  be  calcu- 
lated from  the  screen  reading  in  the  usual  manner.  Another  device 
consists  of  the  use  of  a  special  lens  mounted  in  front  of  a  wedge- 
shaped  screen,  the  lens  being  constructed  so  as  to  reverse  the  image 
of  the  two  sides  of  the  screen,  as  viewed  by  the  eye,  when  such  lens 
is  in  front  of  the  screen.  The  lens  is  so  mounted  that  it  can  be  oscil- 
lated rapidly  in  front  of  the  screen,  giving  the  same  result  as  would  be 
obtained  were  it  possible  to  reverse  the  screen  at  such  a  rapid  rate  as 
to  cause  the  illumination  on  the  two  sides  to  appear  continuous.  The 
setting  of  this  screen  is  accomplished  as  with  the  more  simple  forms. 

Still  another  flicker  photometer,  the  Simmance-Abady,  makes 
use  of  a  rotating  wheel.  This  wheel  is  made  of  a  white  material 
having  a  diffusing  surface,  and  its  edge  is  so  beveled  that  during  part 
of  a  revolution  a  surface  illuminated  by  one  of  the  light  sources  is 
viewed  through  the  eye-piece  of  the  instrument,  and  during  the  other 
part  of  the  revolution  a  surface  viewed  by  the  second  light  source  is 
observed.  The  flicker  occasioned  by  this  change  disappears  when 
the  screen  is  brought  to  a  point  where  it  is  equally  illuminated  by  the 
two  light  sources. 

By  the  use  of  such  forms  of  photometers  it  is  found  that  results 
with  different  colored  lights  can  be  obtained,  which  are  comparable 
with  results  obtained  with  lights  of  the  same  color. 


INDEX 


PART  PAGE 

Alternating-current  circuits I?  40 

calculation  of I,  44 

line  capacity I,  43 

mutual  induction   I,  42 

skin  effect I,  42 

Alternating-current  lines,  drop  in I,  44 

Amyl  acetate  lamp II,  17 

Arc  lamps II,  32 

carbons  for II,  41 

efficiency II,  41 

electric  arc II,  32 

flaming  arc II,  42 

Bremer    II,  42 

magnetite   II,  44 

mechanisms    II,  33 

carbon-feed II,  37 

rod-feed II,  37 

series » II,  35 

shunt  . , II,  35 

rating  of II,  40 

types  of II,  38 

alternating-current    II,  39 

direct-current II,  38 

interchangeable    II,  40 

Arc  light  photometry II,  93 

B 

Bremer  arc  lamp II,  42 

Bushings    I,  65 

C 

Carbon  incandescent  lamps,  manufacture  of II,  3 

Carcel  lamp II,  77 

Concealed  knob  and  tube  wiring I,  13 

Conductors,  calculation  of  sizes  of I,  25 

Cross-arms   : I,  75 

Cut-out  panels I,  66 

i 


ii  INDEX 

E 

PART  PAGE 

Electric  lighting II,  1.94 

arc  lamps II,  32 

arc  light  photometry II,  93 

classification II,  2 

history  and  development II,  1 

illumination II,  53 

incandescent  lamp  photometry .'.  II,  88* 

incandescent  lamps II,  2 

light  standards II,  76 

lighting  of  public  halls,  offices,  etc II,  64 

photometers  : II,  79 

photometry II,  76 

power  distribution II,  46 

residence  lighting II,  56- 

shades  and  reflectors II,  72 

special  lamps II?  27 

street  lighting  II,  67 

Electric  wiring  I,  1-87 

alternating-current  circuits  I,  40 

alternating-current  lines,  drop  in I,  44 

concealed  knob  and  tube  wiring I,  13 

conductors,  calculation  of  sizes I,  25 

methods  of  wiring I,  1 

outlet-boxes,  cut-out  panels,  and  other  accessories I,  63 

overhead  linework ; .  I,  68 

testing  I,  36 

two- wire  and  three-wire  systems I,  20 

underground  linework  I,  79 

wires  run  concealed  in  conduits I,  1 

wires  run  exposed  on  insulators I,  16 

wires  run  in  moulding I,  9 

wiring  installation I,  29 

wiring  an  office  building I,  54 

English  candle II,  76 

F 

Fibre  conduit I,  83 

Fibrous  tubing . I,  15 

Frosted  globes II,  73 

Fuse-boxes   I,  66 

G 

Gem  metallized  filament  lamp II,  12 

German  candle   i II,  77 


INDEX  iii 

H 

PART  PAGE 

Helion  lamp 1I?  21 

Holophane  globes n,  74 

I 

Illumination  n,  53 

intrinsic  brightness II?  54 

irregular  reflection  n,  54 

regular  reflection  n,  54 

unit  of II,  53 

Incandescent  lamp  photometry n,  88 

Incandescent  lamps !' II,  2 

comparison  of  types II?  25 

distribution  of  light II.,  H 

efficiency II,  6 

gem  metallized  filament  lamp II,  12 

helion  lamp  II,  21 

manufacture  of  carbon  incandescent  lamps. II,  3 

mean  spherical  candle-power II,  12 

metallic  filament  lamps II,  14 

Nernst  lamp II,  21 

selection  of  lamps II,  8 

voltage  and  candle-power  ( II,  5 

Insulators I,  77 

L 

Light  standards II,  76 

amyl  acetate  lamp II,  77 

carcel  lamp II,  77 

English  candle II,  76 

German  candle II,  77 

pentane  lamp II,  77 

working  standards II,  78 

Lighting  of  public  halls,  offices,  etc II,  64 

Lightning  arresters I,  78 

Lummer-Brodhun  photometer II,  80 

M 

Mercury  vapor  lamp II,  27 

Metal  conduit,  wires  run  in I,  4 

Metallic  filament  lamps • II,  14 

Moore  tube  light II,  29 

Mutual  induction , , . . I,  42 


iv  INDEX 

N 

PART  PAGE 

Nernst  lamp II,  21 

0 

Opal  enclosing  globes II,  76 

Osmium  lamp   II,  20 

Outlet-boxes I,  63 

Overhead  Hnework   I,  68 

corners    I,  75 

cross-arms    I,  75 

insulators   I,  77 

lamps  on  poles I,  79 

lightning  arresters I,  78 

pins I,  77 

poles   ' I,  70 

guying  of I,  72 

placing  of I,  69 

service  mains,  pole  wiring,  etc I,  77 

P 

Pentane  lamp    II,  77 

Photometers  \ II,  79 

integrating II,  86 

Lummer-Brodhun   II,  80 

portable    II,  85 

Weber  II,  83 

Photometry    II,  76 

arc  light II,  93 

incandescent  lamp II,  88 

light  standards II,  76 

photometers    II,  79 

Poles I,  70 

Polyphase  circuits I,  53 

Power  distribution II,  46 

multiple  or  parallel  systems  of  distribution II,  51 

multiple-series  or  series-multiple  systems II,  51 

multiple-wire  systems II,  53 

series  system II,  46 

E 

Residence  lighting II,  56 

arrangement  of  lamps II,  59 

calculation  of  illumination II,  56 

plan  of  illumination II,  56 

types  of  lamps II,  56 

Rigid  conduit,  wires  run  in I,  1 


INDEX  v 

s 

PART  PAGE 

Shades  and  reflectors H,  72 

frosted  globes II,  73 

holophane  globes II,  74 

opal  enclosing  globes II,  76 

Skin  effect I,  42 

Special  lamps II,  27 

mercury  vapor  lamp II,  27 

Moore  tube  light II,  29 

Street  lighting II,  67 

T 

Table 

absorption  loss  for  different  shades II,  72 

arc  lights  per  mile II,  70 

armored  conductors — types,  dimension,  etc I,  8 

change  in  voltage,  effects  of II,  8 

conductors  in  fibrous  conduit,  sizes  of I,  15 

Cooper-Hewitt  lamps   - II,  65 

drop  in  alternating-current  lines,  data  for  calculating.  I,  47 

efficiency  of  transmission  of  arc  light  globes II,  76 

flaming  arcs,  general  data  on II,  46 

gem  metallized  filament  lamp  data II,  12 

Greenfield  flexible  steel  conduit I,  5 

illuminating  data  for  meridian  lamps II,  62 

intrinsic  brilliancies  in  candle-power  per  square  inch . .  II,  54 

life  of  a  25  c.  p.  unit  data II,  16 

lighting  data  for  arc  lamps II,  66 

melting  point  of  some  metals II,  21 

Moore  tube  light  data II,  32 

mouldings,  sizes  of  required  for  various  sizes  of  con- 
ductors      I,  12 

Nernst  lamp  data II,  25 

photometric  units   II,  78 

pole  data I,  71 

relative  reflecting  power II,  55 

residence  lighting  data 4 II,  63 

rigid,  enameled  conduit — sizes,  dimensions,  etc I,  2 

single  wire  in  conduit I,  2 

standard  vitrified  conduit I,  81 

street-lamp  data II,  71 

tantalum  lamp  data II,  15 

three  wires  in  one  conduit I,  3 

tungsten  lamps  (multiple  and  series) II,  20 

two  wires  in  one  conduit I, 

Testing  electric  wiring I,  36 


vi  INDEX 

....          ,  i. 

PART  PAGE 

Three-wire  system,,  details  of -I,  22 

Tungsten  lamp II,  16 

Two-wire  and  three-wire  systems I,  20 

details  of  three-wire  system .  . . . I,  22 

relative  advantages I,  20 

U 

Underground  linework I?  79 

fibre  conduit   I,  83 

iron  pipe   I,  80 . 

laying  of  conduit I,  82 

vitrified  tile  conduit I,  80 

w 

Weber  photometer II,  83 

Wires  run  concealed  in  conduits I,  1 

armored  cable I,  6 

in  flexible  metal  conduit I,  4 

in  rigid  conduit . I,  1 

Wires  run  exposed  on  insulators I,  16 

accessibility    I,  17 

cheapness I,  16 

durability I,  16 

Wires  run  in  mouldings I,  9 

Wiring  installation I.,  29 

feeders  and  mains. I,  36 

location  of  outlets . I,  30 

method  of  wiring I,  29 

systems  of  wiring I,  30 

Wiring  an  office  building I,  54 

basement .  I,  55 

character  of  load I,  55 

electric  current  supply I,  54 

feeders  and  mains I,  55 

first  floor   I,  58 

interconnection  system I,  58 

second  floor   I,  58 

switchboard I,  55 

upper  floors I,  58 


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FEBs 


MA?  10  ... 

JUN  10  1941 

'49' 


*6 


SEP  2  3 1952  UJ 


1933 


LD  21-50m-l,'3 


-•o 


YC   19518 


346638 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


